Using Flash in a showing / hiding DIV

On a page I am developing right now, I want to have a menu that slides
down when clicked on a handle, like the menu on http://codeigniter.com/user_guide/
I have succeeded in creating such a menu, however, when replacing the
(text) content with a flash movie, the behaviour becomes very odd in
most browsers. Parts of the movie start showing through even when it
is hidden, or the movie won't show at all, even when the menu is
expanded.

To circumvent this problem, I have tried to only display the movie
when the menu has fully expanded, using the following code:

var menuShow = false;
function toggleMenu() {
myHeight.toggle();
if (menuShowing) {

document.getElementById('objectFlashMenu').style.visibility= 'hidden';
menuShowing = false;
} else {

setTimeout("document.getElementById('objectFlashMenu').style .visibility='visible';",
500);
menuShowing = true;
}
}

In Safari 3 on OS X this works fine, but in Firefox or IE7, it
doesn't. Can anyone offer a solution or workaround so that I can
achieve the desired functionality? To see the page, visit
http://10074:BxiO6NFu@www.10074.07ds.thinkquest.nl/

Thanks for any help.