I tested the issue with the latest version of RadEditor for MOSS, which now is 5.6.1 and was not able to reproduce it. For your convenience I attached a small video with one of my tests. Inform me if I am doing anything wrong in my test.
What I suggest you to do is upgrade to the latest version of RadEditor for MOSS. If the problem persists, provide me with your configuration files (ConfigFile.xml, ToolsFile.xml, MOSSEditorTools.js). They are located in the /Program Files/Common Files/Microsoft Shared/web server extensions/wpresources/RadEditorSharePoint/5.x.x.0__1f131a624888eeed/Resources/
folder. A step by step scenario of the problem will be helpful in order to reproduce it as well.
When you upgrade, make sure that you follow the instructions in the following online help article http://www.telerik.com/help/aspnet-ajax/upgrade_to_a_newer_version_of_radeditor_for_moss.html
In order to download the latest version of RadEditor for MOSS you should go to the Downloads section under your account, click on the Visual Studio Extensions, DLLs, Scripts, Source Code, DNN and MOSS wrappers ... link and download the RadEditor for MOSS: (zip, 13 MB) installation. You can see the attached screenshot for more information.
the Telerik team