Don't worry bro, im guessing it's an older computer? I have the same problem, the laptop I play on overheats periodically during the game and drops the game to about 4FPS which dumbed down means very very laggy, like, unable to move and lie down laggy.
Also not as often but it does happen, is mine too just completely shuts off, it's just from overheating. Find where the fans are and make sure they have plenty of air to suck in and make sure where the hot air is escaping is free aswell and not up against a wall or something else blocking it.