There must be some setting error somewhere.. because when i tried playing it in my office computer (it was a 7850 or something) it worked perfectly fine.. no lag, no fps drops.. But then again I might only play for a brief amount of time.

What happens for me in my current computer is that it gets progressively worse the longer I have my computer on, up to 30 fps then it doesn't get any worse. On lonely areas the limit is about 39 fps on HIGH, 58-61 fps on STANDARD desktop. On super crowded fates it will go as low as 11 fps zoomed out.

Here's an overview of my stuff

i5 2.79 GHZ
Powercolor 7970 3GB
Motherboard Intel DH55TC
8GB ram
750W PSU
Monitor samsung Syncmaster BX2240
Driver beta 13.1 Catalyst control center