MacBook Pro :: Goes To Sleep While Playing Games

I'm not a big gamer, just a couple hours a week, perhaps. This particular problem occurs while playing Left 4 Dead 2 and Half Life 2. Basically, the computer just goes to sleep while playing the game. It doesn't seem to hurt anything. Hitting they keys a few times brings the computer back and the game back into play without a hitch. Except the audio sometimes needs to catch up. But it sure is annoying. I haven't actually timed it. But it doesn't seem entirely random. It seems to be at somewhat predictable intervals. Maybe 15-25 minutes. I looked all through my settings (especially Energy Saver), and keyboard shortcuts and it doesn't seem that any of them would cause the computer to sleep. The computer is plugged into the wall.