IE8 dropdown navigation images disappear?

Hi all,

I need help with a magento site. I have added in a drop down menu, which seems to be conflicting with my category pages in IE8 with compatibility mode turned off.

http://www.total-displays.com/portable-displays/pop-up-stands/classic-pop-up-stands.html

If you rollover the left hand category structure, then rollover the displayed category images, you will find these images disappear. To make them reappear again, you need to roll back on the menu.

The following code in a file called menu.js looks like it could be part of the problem. I was wondering if there was anything we can include to allow the menu to drop down, but without affecting the category images!

Menu.js


function toggleMenu(el, over)
{
    if (over) {
        Element.addClassName(el, 'over');
    }
    else {
        Element.removeClassName(el, 'over');
    }
}

Thanks