Hello Daní,
You seem to be having a rather complex scenario. It would be easier for us to reproduce your issue if you prepare a demo and get back to us.
You might also want to check our reply to your other forum post first
http://www.telerik.com/community/forums/silverlight/richtextbox/several-issues.aspx to see if it gives you an idea of a different approach.
Regards,
Iva
the Telerik team
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