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.