Could you please try using ASP:UpdatePanel instead of RadAjax and let me know if the issues persist?
Also, could you elaborate a bit more in the way you use the RadAjaxManager control to ajaxify the web part. Note that the RadAjaxManager should be placed in the application MasterPage and the ajax settings should be added dynamically on Page_PreRender of the web part.
All the best,
the Telerik team
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