There was an issue with RadToolTip's quickly disappearance in some situations, especially when the mouse is moved very fast. Today we release a Service Pack and in it the RadToolTip's performance is improved. I recommend you to download this newest version of the control and test your project with it.
Unfortunately the provided code is not enough for us to find the reason of the problem that you report, but we suppose that it is not caused by the control but by the way of its usage.
We assume that your scenario is very similar to this demo
. Please, check it, try to implement it and if you continue experiencing problems, send us a sample project. As soon as we receive it, we will do our best to help you.
the Telerik team