Single or multi player, after about 20 - 50 minutes the framerate will dive down to almost nothing. I have attempted all settings levels, malloc DLL's, maxmem and maxvram, as well as using fullscreen windowed.

Different combinations effect the standard framerate, but the die-off happens regardless. The framerate stablises somewhat when standing still, but looking around drops it by about 50% and moving drops it further to about 1-2 frames per second

Looking at taskmanager, the memory used drops from about 1.3gb to about 500mb. I believe this is some sort of memory leak or a failed reading of usable memory by the game.