I've had this problem occur at random, intermittently, across boxes at multiple companies, multiple distros, and multiple kernel revs. I think that I'm just cursed.
What happens is that I'll have a new box loaded, and I'll be doing something inside ssh, like a yum update or an apt-get. Everything will be progressing fine and then the session just stops. The ssh doesn't disconnect, it's almost as if someone sent a scroll lock keypress to the session.
If I hit enter, or space, or just about any keystroke in the ssh session, the session wakes back up and its as if nothing went wrong.
I once upon a time tracked down an issue like this and found that the interactive processes were entering SLEEP status. I can't confirm that this is the same behavior in this case, but I will amend the question if I can pinpoint that specific cause.
Regardless, I was never able to figure out why the processes were randomly going into sleep.
Has anyone had this or a similar issue and have any insight into what might be causing it?