I've noticed with Witcher 2 that it likes to read from the HDD a lot. I started getting occasional slowdowns and a very infrequent stutter and a good defrag took care of it. As for the sound you could try uninstalling the HD audio driver through the ATi uninstaller in the Control Panel and install 11.5's. Who knows, your stuttering issue might have something to do with the sound drivers as well. These drivers have been solid for me, no issues and the upper right mouse lag has been fixed and I no longer get a somewhat rare Dirt 3 crash anymore.