Weird "jumping" in menus in IE6

I'm using pseudo-DHTML dropdown menus adapted from an article on A List Apart and it works fine in Firefox (of course), but in IE6 when you rollover the main link, all of the links below it "jump" slightly.

The technique, for those unfamiliar with it, is to change the <LI>'s class on mouseover via Javascript so that the hidden floating menu appears.

You can see it at http://www.jetaasc.org/newsite/index.php and the CSS is at http://www.jetaasc.org/newsite/css/main.css.

Any help would be greatly appreciated. Thanks.