I recommend to download the latest "Futures" build of "Prometheus" controls - you can do this from your Client.net
account .The boundary detection when using RadToolTip is improved there and I suppose that this will solve the problem.
You can also show the RadToolTip on certain event, for example onmouseover, instead of using the Sticky property. It is up to you to choose the most appropriate for your specific case implementation.
the Telerik team