What would be the most likely reason a desktop PC would constantly turn itself on and off every 3-5 seconds? It turns on long enough to spin the fans and HDDs then turns right back off like it's on a timer. It runs well without issue, but hours after turning it off it starts doing that. Different power supplies and replaced the front power switch...continues doing it. I've went through several power settings and disabled anything I could see like "wake on lan" or anything else, but see nothing. I've unplugged the wireless mouse and keyboard and also pulled the lan cable, it still does it.