I checked it, now I know why this happens. When you move the mouse over the tooltip, the MouseLeave event is fired (since you are no more over a control, rather a tooltip box) and the tooltip is hidden. When the tooltip is hidden, then you are back on the control and the MouseHover event is fired displaying the tooltip and this goes in a cycle.
Now that I understand the issue I have to look for a solution.
Thanks for reporting Jean-Marie