I have had this issue occur sometimes but not all. My solution was to simply reload the game and it worked.
How much ram do you have?
What type of video card do you have and are you using the most current drivers?
Also check and see how much virtual ram you have allocated, I advise double whatever your ram is but others here may prefer another setting. If you have 4 gigs or 6 gigs of ram, double your virtual memory to 8 or 12 gigs.
