pc shutting off or rebooting

Hey guys I have a problem, my PC shuts down (and reboots) at fully random intervals with no connection to anything. It happens around 3 times a day, while I'm playing and when it's idle too. It just completely powers off. First, I thought it was the PSU that was doing it, and I replaced it...

Hi, I really have 2 questions, but 1 in particular because I THINK I have this issue pegged down to my rather old pci-e cables. I don't think it could be the PSU itself because its working fine, as far as I can tell. 2 reasons I think it is the cables is because my PC kept just turning off...