[Mstenger404]

if it always locks up after a certain duration, it must be the temperature for sure.

Check to be sure your video card's fan is 100% working. Nvidia control panel should have an option to check that out.

Like sam said, take a can of compressed air and blow any dust out. Then you can make sure all cables are tied and tucked out of the way of major air flow, look for places where components intake air and be sure they're clear.

Also a tricky one is the motherboards chipset, with my last motherboard the video card was so huge it blocked airflow to the northbridge, and covered the southbridge completely. The chipset are some critical components you need to keep an eye on.