I use AjaxManager to refresh the page and get this problem when I add Rich Text Editor in the code.

A script on this page is causing Internet Explorer to run slowly.
If it continues to run, your computer might become unresponive. 

Looking at an earlier forum that there have been problems with the UpdatePanel.
The problem occurs only in Internet Explorer (tried version 8 and 9) and Internet Explorer stops responding...
Works flawlessly in Firefox, Chrome and Safari with Rich Text Editor :-)