This sounds like another problem and we will need more data to analyze it. Can you please take a look at this issue
shared publicly? If that's the same you experience (note, it's specific to RadComboBox for Silverlight) we have a solution implemented in the latest internal build (labeled 2012.1.518
available for download from your account). If that's not the case we will need a test case replicating the problem we can run locally and will get back to you with our findings.
the Telerik team