FPS Issues Q_Q

Intel(R) Core(TM) i5-4590 CPU @ 3.30GHz - CPU
NVIDIA GeForce GTX 960 4 Gig - GPU
8.1 GB - Ram
1 Terabyte HD
Windows 7 Home Premium 64-bit

My fps stays around 120 to 180 when my sync is off, averages 150~. Yet, when I'm in stormwind around the auction house, it drops to around 40ish. I worried about being able to maintain 60 fps in 25 mans with at least medium/ultra graphics. I played back in retail and could barely raid in a 10 man, couldn't afford what I have now. Any idea on why it drops and if I will be able to enjoy the mundane graphics that is a 25 man raid>?
Any help would be appreciated. :D