I would suggest by starting with monitoring the "usage" statistic for both your CPU and GPU. When the frame rate dips, look at your usage and see if either one spikes to near 100%. That's where you issue will be. Currently, there is no consistent bug among players that causes persistent frame drop that isn't related to hardware.
I'm fairly certain all Nvidia cards suffer from FPS drops in C3, AMD cards however don't. My GTX 670 had FPS drops in C3 but my 7970 has no drops at all.