I created a simple page to test your scenario, but could not reproduce the problem - please find it attached. As soon as you change the pageIndex of the RadGrid, the TargetControls collection of the RadToolTipManager becomes empty, but none of the elements on the page gets tooltipified. My tooltips close after 10 seconds in IE7 as well.
Please make sure that you are using the latest version of the RadControls for ASP.NET AJAX suite, and in case the problem persists, we will need a simple running application, demonstrating the problem, so that we can research on our side what is causing it.
the Telerik team