JavaScript hover menu not disappearing when it should

Alright, so I'm trying to write a drop-down menu in JavaScript, but it doesn't vanish when it should (on mouse out). How do I make it vanish when it shou开发者_开发问答ld?

Here is the site it's on: http://prime.programming-designs.com/designs/graymech/about.htm

You have ReferenceError on lines 31 and 43 in your code where a function called IsMouseOver is invoked.

Appears as though this function doesn't exist in your code.