I was recently having problems with choppy frame rate and after some experimentation wanted to share my results.

Rig
i5-3450 (4 @ 3.10GHz)
32GB RAM
2 GeForece 660 Ti in SLI

Problem
Frame rate would vary from solid 60fps in some areas, down to 20-30 in other areas.
Despite FPS dip, perfmon showed no pressure on the machine. (40% CPU, 4GB RAM used)

Solution
The apparent solution was to disable Parallax Occlusion and set SSAA to Low.

Video here: http://www.youtube.com/watch?v=CmRfyIeyNh0