This is very strange. I have a RadMenu inside a User Control that is not visible until the user logs in. However, after the login when the screen redisplays the RadMenu item is about 300px wide with black horizontal lines filling the empty space and the submenus pop out to the right of that area. After the next reload the menu is fine and stays that way from there on out.
The RadMenu uses no text (Text="") and just an ImageUrl (no other images).
The UserControl sits on the master page.
There is a screen capture of the issue here: http://www.rowantree.org/RadMenuProblem.png
Screen Capture Of Issue