We are aware of a similar problem in RadComboBox, which have been resolved. After a couple of hours we will be releasing an update to RadControls for Silverlight. Please, check if the new version resolves your problem too. If it isn't, I will need a sample page, which can be used to reproduce it.
Thank you for your patience and best wishes,
Valeri Hristov (Silverlight Team)
the Telerik team
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