When I first start playing I have really smooth fps. Though it begins to slowly drop over time. After about 15min I have half fps and 30min I have 10 fps or so. What could be causing this and how may I fix it? I've tried playing around with graphics settings, most are on normal. Video Ram is set to highest. Specs: Gtx 580 corei7 950 3.07ghz 6gb ram win7 64bit EDIT: If I restart the game I return to normal fps and repeat the cycle.