ATI burned me a long time ago and I never went back.  I hear about tons of problems with ATI and OpenGL, I know there are a lot of people who experience no problems but it sure seems a common factor when problems like yours pop up.  I vaguely remember there is some selection box setting that can help specifically for ATI people, I don't recall the details.  Pretty sure this is something Ratscloset (John at SM Tech Support team) has suggested to people in the past (search his posts).

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