Hi,
adding the following line at the end of /etc/ssh/sshd_config
IPQoS cs0 cs0
ruled out my freeze/disconnection SSH problems 
Edit: explaination of the problem:
OpenSSH sets the TOS (type Of Service) field in the IP datagram as “lowdelay” for interactive sessions and “throughput” for non-interactive sessions. My router doesn’t handle properly those settings, so I changed them in Cs0, Cs0 (aka 0x00, 0x00) <==> (best effort, best effort) and solved the instability/freeze SSH issues.
Reference:
sshd_config(5) man page
Differentiated services
I’ll mark this post as solved after some days of testing.
Thank you, Kranz