OK, I was having the exact same problem until a week ago when I solved it. It was caused by faulty RAM. It could be caused by overheating, but your temperatures sound in the same area as mine were, and it wasn't that with my PC - I ran it with the case off for a while to check.
Before you run out and buy more RAM, download a program called 'docmemory' which is free from
Simmtester
Running that will diagnose any RAM problem you may have. Magic. I bet you that's your problem.