Sounds like a heat problem, could be power supply.
Open the case, grab your heatsink, and gently but firmly try to rock it. If it moves, that's your problem. If it passes that test . . .
From a cold start, boot into BIOS, find the screen that shows "PC Health" or equivalent, and watch the temperature. If it climbs, shut down and remove/reinstall the heat sink properly.
If no temps in BIOS, use CPUID's Hardware Monitor.