Thank you for the additional info about the issue you are experiencing.
Could you also let me know whether the issue manifest when AJAX is stopped? Additionally, could you please log the traffic of those 'broken' requests with FiddlerCap
and provide a link to the logs or use FireBug (in Firefox) and observe the CSS requests when the skins fail to load and provide me with screenshots depicting the details?
Finally, to make sure this is a valid issue, could you just try upgrading to the latest release of Telerik.Web.UI and test whether the issue exists in the normal circumstances?
the Telerik team