Random Crashing and computer shutdown

Whenever I play GMod for about half an hour, my display shuts down, then my computer turns off. When i turn it back on, I get the ‘Resuming Windows…’ screen, then i log in and GMod is running, but not responding. This happens literally every time I try to play the game. I know my computer is good enough because I’ve had Gmod on it for almost a year (which is as long as I’ve had the computer) but this crashing has been in the last month. I’ve cleaned my install and completely reinstalled the game, but to no avail. Any help?

“Display Driver has stopped responding”?

That’s what I thought it was at first, but no. It happens on my other computer but that doesn’t make it turn off.