Thesis - dropdown nav menu z-index problem

My site, http://beckergasstationcanopy.com, uses the Thesis theme and one of the navigation links has sub-pages under it. Thesis has a built in dropdown navigation menu system for things like this, but in Internet Explorer, this dropdown menu displays underneath the Monoslideshow on pages with Monoslideshows (home and photos). Z-index in the CSS hasn't helped.

Any ideas?