[HELP] Problem with git when using ssh-key deploy authentication method

Hi, I have a private github (free) repository and I’m using a deploy ssh-key for authentication. I set up everything as described here.
Everything works ok, I can clone, pull and push freely (the deploy-key is for reading and writing).

The problem is that more often than not the git command timeout with this error:

$ git push
kex_exchange_identification: read: Operation timed out
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

But, if I repeat it several times (without changing anything) it finally works.

The only diagnostic I’m able to do is run ssh -vvT git@github.com-privtest, It also works only sometimes.

When 'ssh -vvT' works
OpenSSH_8.1p1, LibreSSL 2.7.3
debug1: Reading configuration data /Users/Pereiro/.ssh/config
debug1: /Users/Pereiro/.ssh/config line 1: Applying options for github.com-privtest
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 47: Applying options for *
debug1: Connecting to github.com port 22.
debug1: Connection established.
debug1: identity file /Users/Pereiro/.ssh/privtest.github type 0
debug1: identity file /Users/Pereiro/.ssh/privtest.github-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.1
debug1: Remote protocol version 2.0, remote software version babeld-91825f0b
debug1: no match: babeld-91825f0b
debug1: Authenticating to github.com:22 as 'git'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: local client KEXINIT proposal
debug2: KEX algorithms: curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,ext-info-c
debug2: host key algorithms: rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,ssh-rsa-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-256,ssh-rsa,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519
debug2: ciphers ctos: chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com
debug2: ciphers stoc: chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com
debug2: MACs ctos: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: MACs stoc: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: compression ctos: none,zlib@openssh.com,zlib
debug2: compression stoc: none,zlib@openssh.com,zlib
debug2: languages ctos: 
debug2: languages stoc: 
debug2: first_kex_follows 0 
debug2: reserved 0 
debug2: peer server KEXINIT proposal
debug2: KEX algorithms: curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256
debug2: host key algorithms: rsa-sha2-512,rsa-sha2-256,ssh-rsa,ssh-dss
debug2: ciphers ctos: chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr,aes256-cbc,aes192-cbc,aes128-cbc
debug2: ciphers stoc: chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr,aes256-cbc,aes192-cbc,aes128-cbc
debug2: MACs ctos: hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,hmac-sha1-etm@openssh.com,hmac-sha1
debug2: MACs stoc: hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,hmac-sha1-etm@openssh.com,hmac-sha1
debug2: compression ctos: none
debug2: compression stoc: none
debug2: languages ctos: 
debug2: languages stoc: 
debug2: first_kex_follows 0 
debug2: reserved 0 
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: rsa-sha2-512
debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ssh-rsa SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8
debug1: Host 'github.com' is known and matches the RSA host key.
debug1: Found key in /Users/Pereiro/.ssh/known_hosts:1
debug2: set_newkeys: mode 1
debug1: rekey out after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug2: set_newkeys: mode 0
debug1: rekey in after 134217728 blocks
debug1: Will attempt key: /Users/Pereiro/.ssh/privtest.github RSA SHA256:Arv/O09CMsRlm7zztr58gw4BI/wlVZzbujz31Oe3Qb4 explicit
debug2: pubkey_prepare: done
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<ssh-ed25519-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp256-cert-v01@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,sk-ecdsa-sha2-nistp256@openssh.com,ssh-ed25519,ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256,ssh-rsa,ssh-dss>
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /Users/Pereiro/.ssh/privtest.github RSA SHA256:Arv/O09CMsRlm7zztr58gw4BI/wlVZzbujz31Oe3Qb4 explicit
debug2: we sent a publickey packet, wait for reply
debug1: Server accepts key: /Users/Pereiro/.ssh/privtest.github RSA SHA256:Arv/O09CMsRlm7zztr58gw4BI/wlVZzbujz31Oe3Qb4 explicit
debug1: Authentication succeeded (publickey).
Authenticated to github.com ([140.82.114.3]:22).
debug2: fd 7 setting O_NONBLOCK
debug1: channel 0: new [client-session]
debug2: channel 0: send open
debug1: Entering interactive session.
debug1: pledge: network
debug2: channel_input_open_confirmation: channel 0: callback start
debug2: fd 5 setting TCP_NODELAY
debug2: client_session2_setup: id 0
debug1: Sending environment.
debug1: Sending env LANG = en_US.UTF-8
debug2: channel 0: request env confirm 0
debug2: channel 0: request shell confirm 1
debug2: channel_input_open_confirmation: channel 0: callback done
debug2: channel 0: open confirm rwindow 32000 rmax 35000
debug2: channel_input_status_confirm: type 99 id 0
debug2: shell request accepted on channel 0
debug2: channel 0: rcvd ext data 105
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug2: channel 0: rcvd eof
debug2: channel 0: output open -> drain
debug2: channel 0: rcvd close
debug2: channel 0: chan_shutdown_read (i0 o1 sock -1 wfd 6 efd 8 [write])
debug2: channel 0: input open -> closed
debug2: channel 0: obuf_empty delayed efd 8/(105)
Hi josePereiroDev/PrivTest! You've successfully authenticated, but GitHub does not provide shell access.
debug2: channel 0: written 105 to efd 8
debug2: channel 0: obuf empty
debug2: channel 0: chan_shutdown_write (i3 o1 sock -1 wfd 7 efd 8 [write])
debug2: channel 0: output drain -> closed
debug2: channel 0: almost dead
debug2: channel 0: gc: notify user
debug2: channel 0: gc: user detached
debug2: channel 0: send close
debug2: channel 0: is dead
debug2: channel 0: garbage collecting
debug1: channel 0: free: client-session, nchannels 1
Transferred: sent 3068, received 2756 bytes, in 0.3 seconds
Bytes per second: sent 11821.4, received 10619.2
debug1: Exit status 1
When 'ssh -vvT' doesn't works
OpenSSH_8.1p1, LibreSSL 2.7.3
debug1: Reading configuration data /Users/Pereiro/.ssh/config
debug1: /Users/Pereiro/.ssh/config line 1: Applying options for github.com-privtest
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 47: Applying options for *
debug1: Connecting to github.com port 22.
debug1: Connection established.
debug1: identity file /Users/Pereiro/.ssh/privtest.github type 0
debug1: identity file /Users/Pereiro/.ssh/privtest.github-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.1
debug1: Remote protocol version 2.0, remote software version babeld-91825f0b
debug1: no match: babeld-91825f0b
debug1: Authenticating to github.com:22 as 'git'
debug1: SSH2_MSG_KEXINIT sent
ssh_dispatch_run_fatal: Connection to 140.82.114.4 port 22: Operation timed out

This is my config file at /.ssh/config

Host github.com-privtest
    Hostname github.com
    IdentityFile ~/.ssh/privtest.github
    PreferredAuthentications publickey
    IdentitiesOnly yes
    StrictHostKeyChecking no

My internet connection doesn’t seem to have any particular problem, I can work just fine with other public repos.

My main question is: Why does it work sometimes?

Thanks!!!