I have been trying to figure this one out for a few months now. When I first start Fallout 3 (Steam GOTY version), it runs super fast. As time goes on, the framerate just gets worse and worse to the point that it is unplayable. It takes about 15 minutes to do this. After this point, if I keep playing, certain screens (like the item screen) will freeze and I have to forcefully quit and restart the game.

I have tried using Windows XP and Vista compatibility modes, turned off desktop composition, verified file integrity with Steam and nothing seems to remedy the problem. It seems to be graphic detail independent as I can put the graphics on super low and it will stop end up doing the same thing.

I'm on a Phenom II 555 and a GeForce GTX 260. Running with 3GB of system memory on Windows 7 x64.

Anyone have similar issues?