Hi!
What driver are you on? Because from what i've experienced the new beta drivers from nVIDIA are useless on some cards and make the game unplayable/unstable or what not (my 460GTX & 650M SLI turned into slideshows with the new ones). In other words try rolling back to 320.49(http://www.nvidia.com/Download/Find.aspx?lang=en-us)
PS: someone somewhere on the forums tested that updating to Windows 8.1 also improves performance significantly, so maybe that would help also, provided that the problem is video related :D
Hope this helps
