If the problem exists only in IE8 then this is something specific to the template that you load in the editor. Please, open a support ticket and send the template HTML file for examination. I will examine it and see what is causing the problem in the content.
In the meantime you can disable the RadEditor content filters and see whether the issue still persists:
<telerik:RadEditor ID="RadEditor1" runat="Server" ContentFilters="none"></telerik:RadEditor>
the Telerik team