It's not too hard to believe that CCC is the culprit here.
Any chance it's one of those weird things that has something to do with the shoddy(ish) underhanded 'optimizations' that ATI has been throwing into CCC since a few cards back?
I know less about ATI's cards than Nvidia's I'm afraid. I've only been hearing this lackluster optimization stuff across some message boards since I came into having this Radeon card and I still don't have a firm grasp on what it means.
Still, kudos to you for narrowing it down some, Bladeparker.