Movie Clip disappears on refresh in Internet Explorer

[Unicron]

Hi everyone. I have a group of developers here and none of us can explain this problem. It seems a theoretical impossibility, but it's happening anyway.

I have a flash presentation im working on for my company. You can see it and its problem here-

http://portfolio.yourc areerinfo.com/importfolio.php

The back button clip in the corner goes back to the site that brought them here.
It always works for my at my desk, in every browser.

However- At everyone else's machine, in any version of IE, the back button disappears after the file is cached (after visiting one time). So if they hit refresh in IE, the button is gone. If I delete the cached version from temp folder, the button is back. (I also viewed the cached file with flash player and IE, and the button is still there).

It makes no sense. We all have the same IE version and the same Flash player version. The button is attached the same way as every other element. It seems impossible... like going to see Titanic and the ship never sinks. It should always happen exactly the same.

Heres the simple AS for the button, just like all the other clips:

attachMovie("ba ck","back",100) ;
back._x = Stage.width;
back._y = 0;
back.onPress = function() {
getURL("someurl here");
}

That's it, nothing complicated. Yet it disappears if it is cached...