It sounded from when you were talking like you had a false sensor reading. The system isn’t over heating but it thinks it is so it shuts down. The only other thing to do would be a diagnostic scan of the OS for problems there which we wouldn’t be able to diagnose without seeing the machine. If it’s fairly regular, boot up a live CD and see if it still shuts down after it works for a while. If it doesn’t, it’s a Windows problem, if it does, its a hardware problem.
