Hmm its strange. I have similar setup (I5 4670K - GTX 660 TI OC - 8GB) and running @60 fps constantly. With all set on "ultra" in game menu, and on Nvidia control panel. With texture packs, blablablabla.
Wich version of game are you using?
were did you downloaded from?
Are you sure you haven't any demanding program open?
Its not really a strange case cuz this game is bugged as f***. I would recomend installing a clean version (2.2.1 mwm)