LAG Spike During Games On Windows 7 Laptop

For some odd reason, whenever I play any game, it starts off crisp smooth and then maybe 10-20 minutes later it goes completely laggy and then about 20 minutes later, it goes back to smooth gameplay. Some instances of this is when I play Star Wars Battlefront 2, Battlefield 2, ArmA II, Call of Duty Modern Warfare 2. I have tried to set the game's priority to High but it does very little improvment and I have tried Game Booster aswell.