Yeah, as above. At the beginning of your root movie set _level0.loaded=0; In your movie loading in on level1 on the first frames set up a loop with the main script -

_level1.loadedBytes = _level1.getBytesLoaded();
_level1.totalBytes = _level1.getBytesTotal();
if (_level1.loadedBytes<_level1.totalbytes) {
gotoAndPlay ("loop");
} else {

where loop is the label for the previous frame. This will return the variable _level0.loaded to 1 when the movie has loaded. Obviously your button script will probably read

if (_level0.loaded==0){

This will only load the movie if it has not been entirely loaded already.

I'm not sure exactly what your trying to do, you might have to take the stop(); out if you want your loaded movie to play and replace it with a gotoAndPlay

Anyway...Hopefully, this will point you in the right direction and you can work the rest out for youself .

Let me know if you have any probs.