Is the RadComboBox on top a Silverlight RadComboBox from "RadControls for Silverlight". It looks like your page is sliced in parts and the navigation is with ASP.NET control and inside you have Silverlight plugged in. If the combo is not from the telerik's Silverlight suite we may have to change the product of the thread.
Also have you tried to set the windowless = true for your Silverlight plugin? Add the following in the silverlight's html:
<param name="windowless" value="true"/>
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