WoW is most dependent on the CPU for its performance, especially in raids and encounters that have a lot of players and NPCs
One thing you could try to do it set WoW to use 4x Anti-Aliasing and 8x Anisotropic Filtering. This will prevent your GPU from downclocking (if it is what's causing the issue), which was an annoying issue I had when I last played. Also, stick shadows on Low, or whatever the setting's called, so that it's not using Dynamic Shadows. They really hurt the CPU afaik, especially in raids where the Dynamic Lighting and Shadows are to the extreme.