This occurs with the latest versions of FF, IE7, Opera, and Safari. The attached image shows the dropdown items (all "DUMMY") falling behind the lower menu ("FORUM").
I don't think that this is a CSS overflow issue, as menu items further down on the list start appearing later on the page after the menu that masks several of the earlier items.
Again, this is not restricted to one browser, but appears with all the aforementioned browsers.
I am using a custom zen-based theme. The menus are in separate panels2 mini-panels in the header region of the page.
All Drupal modules are the latest available.
Thanks for any clues.