HELP!? My Computer Keeps Turning Off?

i have a Compaq laptop and whenever i play the Sims 3 it turns off after about half an hour. first of all i know the computer meets all of the games requirements, and i have had the game for about 2 years and it has always been fine. now whenever i play i have the laptop plugged in and fully charged, it also sits on top of a special laptop desk so it shouldn't overheat. there was a time where i didn't play the game for about 3 months and now that its summer the last few times i have gone to play it it randomly shuts off, i can hear the music for about 30 seconds and then it just goes in some half off state and i have to hold the power button to turn it completely off. this only happens when i play my game and i'm just wondering if anyone knows whats wrong?