I have had the same problem, I was down 55 fps to 5fps on 2 of my 5 man team after around 30 mins play in the Klaxxi area of wow, I uninstalled my drivers and reinstalled using the custom installation of the nvidia installer for a clean install.
It seems to have resolved my issues, I ran around for 30 mins taking in lots of areas at speed and never dropped below 45 fps
I used Open Hardware Monitor to watch my loading and vram usage, LOad on my Vcore gpu maxed at 87%, Vram maxed at 96.7% (tho most of the time Vram was around 75% average.)

I have GTX 670 SLI 2gb, Vram seems to have been the only issue for me , I will most likely go for a 4gb Vram setup at a later date.