What version of RadControls for Silverlight do you use? This problem existed in the official 2011 Q2 release and should be fixed in the latest internal build. Could you please check does the problem is solved for you in the current release? If not - these fixes will be released officially at the end of the month.
All the best,
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>