Is there a correct way of continuously updating tool-tips?

I've got a graph which is constantly updating and scrolling from right to left. The OnMouseMove sets a trackmouseevent to fire on hover and also causes the tooltip to be activate and to initially generate the tool tip. The OnHover function also causes the function that updates the tooltip with the correct text. The text updates quickly while the mouse is moving.

However the Tool Tip doesn't activate like I planned it when hovering. Instead it updates every few seconds with a burst of activity - two or three updates in quick succession but ~3 seconds with no updates. How can I make it quickly update whatever is under the mouse even if the mouse is still?

Other info:
Currently I have a TRACKMOUSEEVENT structure set every time it enters OnMouseMove and the delay is set to 1 millisecond. Changing this has no effect.

The tool tip class has been extended to allow for tracking tooltips.

Both OnMouseMove and OnMouseHover call the same function that generates the correct text for the tooltip.

Thanks.