I'm attempting to have level3 and beyond of my drop-down menu overlap slightly. It looks great in Firefox, Safari, Chrome, etc. but not IE6/7/8. I've played around with the z-index for different elements, but I have no idea what an appropriate fix for IE might be. I've googled the issue and can't seem to find any solution. Anyone have experience with this before?

The site is based on the Apple Zen template, but that's not significant as it seems to be a CSS compatibility issue.

View the site here:
http://www.atsresources.org/shop

Thank you!