Drop down make content disappear

Here is site I am working on:
https://www.timeforbelfast.org

I have drop downs using css and the css drop down plugin. On the homepage, I have a quicktime movie that is embedded into the index.php file so that it is always at the top.

When you get the drop down on the menu, the movie partly disappears. BUT, I’ve noticed, and this is odd, that if you click to another tab (using firefox) and then back to that tab again, the drop down works fine.

Any way I can avoid this? Or why is this happening?

Thanks