I've had issues solo mining via a Windows client with multiple machines as well, just FYI. 2 machines was fine...but beyond that things would jam up. Could have been an easy fix (I'm not sure)...but I just tossed everything in an Ubuntu VM instead and haven't had an issue since.
Its already been concluded by the resident thread experts that it is not possible for there to be something up with the windows client. It's your hardware at fault
All I'm saying is check the usual suspects before blaming the client. We had a cheap Asrock MB running 4x 7950's with a 1500w Rosewill gold supply. It would just stop mining every few hours and require a restart. Didn't make any difference if you were solo, or pool mining. Swapped the MB out with an MSI, the problem was immediately solved.
I have the exact same issue with one of my machines on an ASRock -- 5x7970. Sometimes it runs for a day before requiring a restart...sometimes 5 minutes. I was blaming it on a ghost driver issue...so wiped and reinstalled xubuntu...set it all back up...same deal.
I'll try swapping out the mb I guess.
This same MB would run flawlessly with 3 cards. We even tried swapping all the cards, the PSU, HD, and the memory. Didn't make any difference if the cards were at stock speeds, or OCed. It was funny, because we had recently read a review saying this was a great MB for mining setups. It simply was not stable with more than 3 cards. YMMV...