You problem is probly overheating and dust build-up
Like Tarkus said, its probly heat and dust build-up, first of all clear out any dust. Re-boot and see if that works any better, if it still crashes, leave your case open to allow air flow, if that doesnt work any better, try put a desk fan aiming into or near your computer to help it cool down, i assume that this will work because i had a similar problem.
One strange problem i had was when i had an Over-heating moniter, if it over heated i had to turn it off for like 30mins, pain-in-the-****. So i just put a single speed fan on it, and a 3 speed fan on the highest setting, then it worked fine.
Apart from the above i have no other suggestions!
Hope this helps
Ph30nIX