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The tooltip will override the onmouseout event of the item. You should avoid binding onmouseout event to the element that has the onmouseover="tooltip.open(this, '...')" event.
You can try this:
var item = "<span onmouseout=mouseMovedFromItem(" + eventId + ")><img class='thumb' ...
+ " onmouseover=createTooltip(this," + eventId + ")"
+ " onClick=fly(" + eventId + ")"
+ " /></span>";