I get the dead ssh sessions syndrome at least 3-10 times a day. (once today during a 83GB file move, thanks screen!)
Then *just* before I am sure they are completely dead, they start responding (thanks keepalive!) and perhaps lose 1-4 of my 30+ sessions.
I use MTR to see when and where the issues are, I use it to tell me if its the Head-End problem or elsewhere.. Lately its been from my DOCSIS router/headend (first hop from the Cable Modem) and DNS lookup issues.
Sorry, context switches are very quick for me lately, sorry if you didn't follow.