I tested the problem with the latest version of RadEditor for MOSS, which now is 5.8.10 and I was not able to reproduce it.
Could you confirm that you are using the latest version of the control. If not I recommend you upgrade to it. You can find the upgrade instructions in the following online help articles:
Upgrade Instructions from version 4.x to version 5.x
Upgrading to a newer version of RadEditor for MOSS
the Telerik team