This seems like an unusual scenario for the RadComboBox. It does not happen in our examples, so it must be something specific to your project.
Could you provide more information about what's on the page, what styles / scripts are loaded, how big is the combo, as well as information about the version of the controls that you are using?
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed