I'm a fairly technologically savvy person but it's always nice to hear ideas from others.
My roommate's PC started turning off randomly since last night. He woke up this morning with it off(we usually leave our PC's on overnight) but thought nothing of it, and today during a game it turned off again.
Then it goes through a random cycle of: not turning on at all, turning on for a few minutes, turning on for a few seconds, etc.
Last night, we got home from a 4day vacation where neither of our PC's were on and I checked sensors and overheating is out of the question for this one. My thought is that the power supply may be too weak and over time has lost the ability to power everything optimally.