Did you download the latest driver from Wacom's site (version Driver 6.3.3-3 for Mac OS X v. 10.8, 10.7 & 10.6)??
Let us know if that fixes the issue.
I downloaded the driver 6.3.3-3 for Mac OS X v. 10.8, but no progress.
I might add that this issue happens with the smudge brush and the mixer brushed, i.e. in jobs that requires "painting". The results on the image after the stokes tend be be very slow. From time to time I "purged>All" and the this solved the problem partly but for a short time.
I think ths may be an issue to work out between Apple (AMD RADEN driver, HD 6770), Adobe (Photoshop CS6) and even Wacom.. I am not an expert, far from it, but perhaps a three way monitoring of the behaviour of the system (Apple, Adobe, Wacom) may shed better light and amore ample perspective on the way to solve those issues.
Thank God I don't have to use the mixer brush that ofter, although I do use the Smudge tool a ittle more frequently.
Let's see if any of you guys has something to contribute to this.
Not at my computer right now so can't be real specific. But, go to the brush pallet menu and at the bottom right(?) where the brush spread example is shown, can't remember name, try adjusting the spread and see if this helps. Had a similar problem in the early days of PSCS 6, and adjusted the spread and problem was solved.
If I get home early enough today I'll try to get more info. Hope this helps.