I'm experiencing some pretty annoying framerate slowdown on basically any medium+ sized map, generally about 30 minutes in... like to the point that the game becomes nearly unplayable, under 10 fps.  I'm on a new computer, all my drivers are updated etc..  Specs are:
AMD Phenom 9600 quad core 2.3 ghz
Nvidia 8800 gt w/ 512 megs
3 gigs of ram
Windows Vista 32 bit

I can run Crysis in High detail high resolution with very little slowdown, Gears of war with everything maxxed etc.  But this makes my machine chug for some reason.  Turning all the detail from high to low makes no difference whatsoever.  No antialiasing, vsync off...  I had AVG running in the background at one point, but I since uninstalled it, to no avail.  Other than that no background apps. 

I DON'T KNOW WHAT TO DO!  I've tried quite a few things, nothing has worked...  Sins is incredible, but it's almost painful to play because of how it runs atm.  HALP!