|Memtest, definitely yes. I prefer together, then separately if errors. no errors are acceptable.|
If heat sink was disturbed, definitely reset right.
FULL spec please. Power supply make, model, and ALL numbers (wattage AND list 3.3V, 5V, & 12V Amps on all rails separately) and all components installed, especially graphics card make and model. List the temps.
Try booting into BIOS set up to see if that is a problem and check temps, etc. there at start up, wait a few minutes to make sure they are stable, then boot to windows.
You have to be a little bit crazy to keep you from going insane.