Game Performance Randomly Sucks

Game installs and loads fine. I can get into the game and start playing fine. After about 2-5 mins of game play my FPS dies and goes to .8 with 1200+ MS when playing in single player. This lasts for between 5-15 mins and then game returns to 45+ fps and 100-200MS. 2-5 Mins after that it happens again. I tried Disabling sound via /nosound on shortcut. made sure video, audio and mb drivers are up to date. Updated Game. Still happening. I'm at a loss. please help.