mouseout trouble in Mozilla

I'm trying to build a dynamic menu, consisting of nested <ul>s, where
each submenu <ul> pops up when the mouse hovers over the parent <li>.
The item <li> the mouse hovers on is highlighted, as well as the
parent <li> of the submenu you're viewing.

I wrote clean HTML, and have the onmouseover and onmouseout attributes
added by a JS function. CSS styles everything.

At http://www.phys.uu.nl/~gdevries/test...ses/test1.html
you'll find a small test page I made with minimal code.

Everything works fine in Opera and IE, but Netscape 7, Mozilla and
Firefox don't behave quite as expected. When the mouse leaves one of
the submenus, it doesn't always disappear. Also, some items stay
highlighted even on mouseout. Strangely enough, this seems to depend
on the mouse speed...

Am I doing something wrong in my code, or is it something in Mozilla's
way of event handling? Any remedies?

Any advice would be greatly appreciated.

Best regards,
Garmt de Vries.