A simple hardware troubleshooting step when your computer is locking up or spontaneously restarting, especially during the boot process, is to reseat the expansion cards in your computer.
Reseating, which is just another way of saying "remove-and-reinsert," assures that the card is making a good, clean connection with motherboard. Expansion cards can work loose and get dirty over time so the simple act of removing and reinserting them in their slots on the motherboard will often clear up what might have looked like a major issue.
Reseating the expansion cards in your PC is pretty easy, even if you've never been inside your computer before. It's certainly worth a try before buying new hardware or hauling your computer in for service.
Have you ever fixed a problem by reseating?