And you have all the patches available for the game? I know that's general stuff but I've no real idea what might be your problem, as I haven't played so I don't know how much of a resource hog it is.

Have you checked your computer on something like "Can You Run It?" to see how it compares to the reccomended hardware. Seeing as it runs at all I can assume you meet the minimum standards but it might be interesting to campare your graphics card performance against whatever is the recommended one. you might be forced to switch everything down to low if you only just rate above the minimum, though your computer sounds like it should be ok.