Are you running Vista? There are either CPU temp gadgets or a lot of high end boards have software utilities that could tell you if you are running hot. Otherwise get into your BIOS and look for temperature readings. If it is a manufactured system (like a Dell) it may have some diagnostic stuff that can run from BIOS. That could rule out memory and perhaps your PS (the fan at least). In my experience it sounds like a heat problem, but never rule the PS out.