I would verify in game settings and also check to see if either your video card software or other software is throttling. I don't know if windows power settings has any options you may want to check but I would make sure sure everything is in full/max power setting until you resolve.

The only other 3rd party software that I know for sure which can do that (besides ISboxer which is banned for WoW these days) would be MSI Afterburner and it's associated RIVA Tuner. Basically I would make sure that and any other 3rd party software is turned off until you resolve it.

Good luck. Please report back if/when you find what is causing your issue.