Well, the only question that remains is what version of the driver you are using?
"Rolling back" to earlier versions sometimes does the trick as "newer" in this case doesn't automatically mean "better", I'm afraid. Looking at the Graphic Hardware Pages
for Softimage some Radeons were tested, not yours sadly, but the testing was done with "AMD Catalyst 11.6 [8.861.0.0]", so while I admit it being a long shot, you might want to try that one... I even found a download location
(assuming you're using Vista x64/Windows 7 x64) as the Autodesk location doesn't work anymore for some reason. But again: a long shot!!!
Did I already mention the "try at your own risk" rule?