If its in a cheakerboard patteren I would say that its a video driver issue. The textures are not being loaded properly. Make sure you have up-to-date drivers. You might also try reinstalling the game, but hold off on that just yet.
If the boxes are kind of flickering in and out and moving on the screen (not just the ground) then you may be having an overheat issue. Its hard to tell because "black boxes on the ground" is kind of a vauge explination.
If you could take a screenshot or something I might be able to help you better.