Are you on the latest tip? Jules added a workaround for this a while back (it still manifests itself occasionally but is much better), and I believe Fabian added a workaround to a somewhat related automation issue in Logic (Steppy automation recording on OSX).
Make sure you aren’t calling repaint() unnecessarily…if you’re using the Slider class hooked up correctly, then dragging the mouse should trigger a setValue() call, which will call repaint() as necessary.