List Items Disappear AFTER Scrolling Over in Navigation Bar. Frustration Factor: High

I'm trying to get this navigation bar on the left to work properly. It is displaying fine and lines up. The problem comes when scrolling up and down the first menu and the previous items disappears. For example, when hovering from straight grinder to angle grinder, the straight grinder text vanishes in IE. Works well in Firefox and Safari though.

http://029c92a.netsolhost.com/brushes.html

Any thoughts, greatly appreciated. Best, Colin