Hi and welcome to the sevenforums, mcvickar
Restarting seems like it could be something else than overheating as well. Usually when overheating it will shutdown and not restart, at least that is my experience.
What could be happening here is you might be getting BSODs, but the system is set for automatic restart. Do you have a .dmp file in c:\windows\minidump? If folder doesn't exist, this means that you're not getting blue screens.