I would agree with zeedee-h and expect it to be a bitmap issue maybe corrupted. Download the demo from here:
http://www.macgamefiles.com/detail.php?item=18748
If you dont have the issue with the demo, then its a good chance you got a game disk with some bad or corrupted software and reinstalling won't remove the issue.
If you still have the issue even with the demo, then maybe I'm guessing its a graphics card issue. Do you have one of the new iMacs with graphic issues? Are you running an old mac with a flashed card? Are you running halo for the pc thru bootcamp or parallels? Are you overclocking your GPU? etc...