I also have the same issue. Some more observations:
I have 5 slugs, 2 used to be uNSLUng, all are now running Debian (as of 2 weeks ago).
I had the problem without any overclocking on the slugs.
They are all overclocked now and the behaviour is the same.
Does not appear to be application-spcific. Sometimes a session just hangs during a period of inactivity which I'll notice it when I try to start typing again. Sometimes I'm compiling or editing a file. It seems to happen at just about any time.
I've noticed it with SSH (Linux/OpenSSH -> SLUG or Windows/X-Manager terminal -> SLUG) and with X-Manager's SCP.
If I am willing to wait, I've found that a session will usually come back after a while.
The slug is not unreachable at the time - you can open up another session very easily, and all other sessions already open are still responsive.