Had this problem in the beta (which I completely removed before installing the full game)

Video showing the problem: http://www.youtube.com/watch?v=KX_Qz3BwqHk

If I use the option to limit the foreground framerate to something between 50 and 100, it will randomly drop down to 50. If I limit to something between 33 and 50, it will randomly drop to 33.

Win7 64-bit
Core i5 2500k @ 4.1GHz
NVIDIA GTX 570 (driver 301.24, the problem has existed over several versions)
8GB ram
Game is installed on SSD

dxdiag: http://pastebin.com/VGCEw0WQ
Edited by jotun#1935 on 5/15/2012 6:09 AM PDT