I had exactly this problem a little while ago and then again earlier today. I was given the following advice originally and it worked at that time. For some reason, it again manifested itself as a problem today. I reversed the advice in my files and now find that it is again working correctly.
The site I had problems with the dropdown menu is - www.zest-uk.net
The advice from Webassist was this:-
in the CSSMenuWriter/cssmw/menu.js file, find the following line:
if(navigator.appName == 'Microsoft Internet Explorer' && cssmw.ieVersion() <= 7) {
and change it to:
if(navigator.appName == 'Microsoft Internet Explorer' && cssmw.ieVersion() < 7) {
Hope this helps,
David