I think this is a known bug in the rotator code that repaints the control. In your scenario, a repaint is forced each time you open/close a panelbar item. The bug is already resolved and the fix will be included in the next RadControls for ASP.NET AJAX service pack release, which is scheduled for next week. If you cannot wait until then, please open a formal support ticket and request a custom hotfix build to be sent to you. I apologize for the inconvenience.
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