Improving framerate with some CS

[serbusfish]

I have a decent computer which in theory should run this game at a constant 60fps, the standard game does actually run at that, but I find in some custom stories I experience a lot of jerky moments were fps drops to below 50 (e.g, Gary Dark Secrets, Followed by Death, and Insanity Nocturnal Confusion).

I have tried lowering SSAO quality to medium and samples to 8, but this doesnt seem to have helped. Is there any way to get CS to run smoothly at 60fps? My system specs:

AMD Phenom II X6 1090T @ 3.8ghz
Zotac GTX 670 AMP! Edition
8GB DDR3 RAM (9 9 9 24)