CSS popup navigation - works with FF and Opera but quirky with IE

Hi,

I've created a popup navigation for a site that I'm building. The page can
be seen here http://www.red36.net/finn/navtest3.htm it works fine in both
Firefox and Opera, but with IE, the parent nav links 'shine' through the
submenu, making the submenu disappear. For example if you are on commercial,
the submenu pops up, but if you scroll down to litigation (with your mouse
cursor on the left hand side of the submenu) the pop up disappears because
the focus is on the private clients.

This menu is based on the vertical menu example at
http://solardreamstudios.com/learn/css/cssmenus/

I've tried adding a z-index, and changing the background colour, but it
doesn't seem to make any difference.

Any help would be appreciated.

Thanks in advance.

Cheers

Nick