Well, for starters, I see document.all in there. Which means you have been reading obsolete books and/or code. Not that it's wrong, per se, just that it's not the best way.
ANYWAY...you don't show how you activate the ShowText and HideText.
And obvious fix is to use onmouseover of whatever-you-are-using to show the box and then onmouseout *OF THE BOX* to hide it. Don't assign an onmouseout for the whatever-you-are-using.
Oh...and there is a MUCH easier way to make a popup box appear near the point of the onmouseover. Without needing to track the cursor all over the screen.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.