Random Shut Down

[HUMAN1ST1C]

This has been aggravating me for the last week: Every time I start a game, everything runs fine, then my computer randomly shuts down after 5-10 minutes. I am having trouble diagnosing the problem because there doesn't appear to be any issues leading up to it. Usually I can tell if something is overheating, or running hotter/louder than usual, but I get nothing.

I'm thinking it might be a power supply issue - any suggestions or insight would be great. This only happens when I start a game up - I've watched movies and TV, and kept my computer on for long periods of time to isolate the issue - and it only happens after I load up a game and start playing.