hidden tooltip but not forgotten

Hi on my page the tooltips are hidden unless triggered thanks to the css code:
visibility: hidden;

However there appears to be a space in ie and firefox where they are being stacked though hidden at the bottom of the page.

Is there a way around this or shoulf this not be happening.

Obviously I could add:
display:none;
to the class which sort the space problem but means the tooltip doesn't dispaly when triggered.

Any help appreciated.

That fixed it, cheers Angus great work.