Powersave mode when playing a game?

Whenever I play games like Assassin's Creed or Prince of Persia even on minimum resolution and settings, my computer abruptly goes into power save mode and I can't get out of it other than just holding the power button until the computer shuts off that way. There are no signs before it happens like drop in frame rate, but there is a soft "clunk/thud" sound immediately after it goes into power save. My computer more than meets the recommended requirements for these games, but the fans are a little louder than usual when I run a game. The crash happens about 5 minutes into a game (runs longer if its not full screen) but can happen even if I'm on a settings page and the game is running. I don't recall if ever happening when the game is minimized and in the background and this only happens in games, nothing else... Overheating maybe? Please help!