Since it's intermittent, it leads me to suspect external causes. Check to see if there are any other programs running that you could close to free up resources, and make sure that nothing decides to auto-update while you're trying to play.
Also, what specific video card are you running? I have heard that certain, specific ones can have seemingly random issues or bottlenecks. Google it, and see if anybody else is having trouble. And what's the processor in your computer?
Sorry I couldn't be more help, but be patient, give lots of information when requested, and be patient while the smart guys work on this. (hmm, semi-ninja'd)