The game is just that poorly optimized for PC. You should feel lucky getting the framerate you're getting, even if it is on a disproportionately powerful PC. I get about 30 fps average on maxed settings on a 2500k \ HD 6950 \ 8GB \ Win7, which is significantly less powerful than your PC but should still chew through the average game from 2008 as well.
One thing that may help if you're that serious about your framerate is turning off Vsync (if you even have it on) to keep the dips slightly higher. Turning all settings to High rather than Very High also gives you a massive framerate gain (much bigger than the graphics actually drop in quality because of it), but that would probably hurt your pride as owner of such a powerful gaming PC :p