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
