ARMA II automatically renders a certain amount of FPS ahead, i have no idea why though. BUT you can change the settings, so that the game only renders the frame you're actually seeing. to do this go to c:\users\USERNAME\my documents\ARMA 2 and open the file arma2ao.cfg search for the value GPU_DetectedFramesAhead=300 (it means the game always renders 300 frames ahead) and change it to GPU_DetectedFramesAhead=1 that should help you with some of the FPS problems.