I have bouhgt a MSI GTX 670 and I can play in very high settings with fxaa or smaa x2 in 1080 p. But in some scenes and some maps while I am getting 65 fps it suddenly drops to 35-40 especially while firing. Why was this occuring ? I tried d3doverrider and somewhat fixed it but not totally. Is there a bottleneck in my pc ? here is my pc specs :

Fx 6100 @ 4.6 GHZ -- GIGABYTE 970A --MSI GTX 670 POWER EDITION -- SANDISK EXTREME 120 GB SSD -- 500 GB SEAGATE HDD -- 8 GB RAM -- COOLERMASTER HYPER 212 EVO CPU Cooler -- AEROCOOL VS-3 CASE -- 500 WATT XIGMATEK PSU -- VE 2 ARCTIC COOLING + 2 AKASA CASE FANS -- 27" ASUS VE276Q LED MON?T?R