Got problems here too.
frist computer: in places framerate drops to 1 fps and screen freezes, this is running at the lowest settings, tried changing setting from high to low but problem still persists
Windows 7 64-bit
AMD Phenom 9600 Quad-Core 2.30GHz
4 GB RAM
Radeon HD5750 512MB GDDR5
second computer: in places framerate drops to about 10 fps, doesn't seem to matter if I have high or low settings.
Windows 7 64-bit
AMD Athlon II X£ 435 Processor 2.89GHz
6 GB RAM
Radeon HD6850 1GB GDDR5