The disappearing menu trick

Not really a trick - more like an incredible annoyance!

I have this groovy menu that slides into the screen when the mouse rolls over a little piece that sticks out. When the mouse rolls off it returns to it’s spot “off screen”. My problem is that sometimes the whole menu gets “lost”.

I have posted the file here:

If you put your mouse over the words “atlas of california” at the left side of the screen you’ll get the navigation bar (and an error probably since I didn’t include the xml file that loads the menu choices). Anyway, works great, but if you zoom your mouse back and forth a few times you lose it. Can anyone tell me why???

The design was copied (with permission) from this site:

Can anyone help me figure this out?