To avoid issues with session timeout use
screen
Run a screen session, detach, return to it later (attach).
There are, of course, other alternatives and utilities, but i find screen mature, so i recommend it.
Is it a real pain to try out BSD nc variant to check if you are having same symptoms ?
To really detected what is happening one would need to tcpdump (or equivalent) and examine the generated files after, looking for suspected behavior.
I see no other way of telling what is going on, if network level is the problem of some sort.
When did the problem surfaced ?
Did it stop working in some point after working for some time or ?
Hope that helps
Regards
Peasant.