Hi I have the same problem before updating the game ran at 65fps and now after a few minutes played FPS starts to fall to 10-20fps.
MY PERSONAL COMPUTER
Benq xl2420t 120Hz
Intel Core i5-2500K
ASUS P8P67-M
Hdd: 1T
8192MB
NVIDIA Gigabyte GTX570
Seasonic 620W Bronze