I'm making a website for my wife's new business, and she wants a horizontal drop-down menu. After looking over many tutorials, I put one together. It works well in some of the browsers I've tested (FF, Opera) but breaks in IE and Google Chrome.

For some reason in Chrome/IE, the drop down expands, but vanishes when I try to mouse onto it. I suspect some sort of miniature gap between the main level items and the drop-down items. Has anyone else experienced this?

Here's an example:
Xpress Dance Classes