I have also this issue in multiplayer sessions after 1,5 to 2 hours. At the beginning i have with my iMac 21,5 (2011 / Bootcamp Win7 64bit native Res.) 30-40 fps with some tweaks in the config file as well in the steam starting options. After 2 hours i get just around 15-20 fps. A friend has a better pc but gets nearly at the same time this issue ^^. Thats why we make and play self-made Missions ;-)