Hmmm... Here would be my guesses.

1)Do you have DirectX9.0c installed? That would be my first guess.
2)Try re-installing your video card drivers, making sure you have the latest ones.
3) Update to A6.31.4 if you haven't already. Also, possibly a file was corrupt on your last update or install, so you could re-download the update and then re-update it and see if that works.
4)You might try to change your refresh rate on your monitor.

Thats where I would start.