First of all, please make sure that you are using the latest version of the Prometheus suite, namely version 2007.3.1314.20.
In case this does not solve the problem, please check whether you have a RadGrid (RadGrid classic) with EnableAjax set to true in one of your usercontrols and whether the problem persists if you disable the ajax of the grid. In case this solves the problem, please disable the ajax of the RadGird and place both controls (RadGrid and RadToolTipManager) in an UpdatePanel with UpdateMode set to "Conditional".
You can find additional information in our online documentation
. In case my assumptions are not correct, we will really need a simple running project with which to investigate what is causing the problem.
the Telerik team