I had exactly the same problem with my 4 x 280X rig, would just hang after a few hours for no reason what so ever. Showed the same showed the same symptoms as your getting.
In the end after a lot of looking around the problem was caused by the IP Stack falling down, no idea why or what caused it. One day it was working fine then all of a sudden it started to play up
I carried out all the same step that you have, reformat of USB key, new USB key, etc.. and still nothing, to the point where I set my rig to reboot every 4hrs to try and keep mining whilst I investigated the problem.
The resolution was to hardcode the IP into the rig and remove the network managers, I'm hunting for the link that I found with all the detail on it but can't find it right now but the command was something along the lines of
apt-get remove network-manager*
Hope this helps, and if I can find the link I will post it up for you