I have seen this behavior only in IE7 and only in case you have a RadSplitter with a RadPane with ContentUrl set to page, holding another RadSplitter (Width/Height to 100%) with RadSlidingPanes. And the "dancing" is only when you try to open the RadSlidingPanes in the nested splitter. If this is your case as well, please add the following style to the HEAD of your child page:
|html, body, form
Otherwise, please prepare and send us a simple running project, demonstrating the described problem.
the Telerik team