Make sure you let memtest run for a few cycles. Sounds like it could also be an overheating problem, first off get motherboard monitor or something similar so you can see your CPU temperature(s). Depending on what you have for a video card it may or may not have a panel for checking the temperature so you may need to download an application to do so. Basically you want to have the temps visible or logging to a file and do exactly whatever it is that makes your PC crash to see what the temperatures were at the time of the crash.
These are the first things to check although it could still be a number of other things, however it definitely sounds like a hardware problem.