Does it happen after extended play? Maybe it's a heat-related issue. Just guessing. When I had a similar problem, except worse (and prettier. so psychedellic! All those beautiful colors and designs
) it
was due to my graphics card, which
was crappy (and no, I'm not asterisk-girl in disguise). But I guess it may be something other that that for you.
One thing I did notice when I upgraded to a better graphics card, was that if I tried to set the resolution too high (past what the game sets it to) crazy weird things happened. Blue blocks everywhere. So I just use the default setting.