AS2 :: Load External SWF Correctly To Main Flash File As Root And Level0 Using LoadMovie()?

I have 2 flash files,one is mine (the main container) and one is a external swf.I want to load the external SWF to my container SWF.I write this code in my AS2 container, inside the main timeline flash (mine flash).In the first method, I add a movie clip with instance name mc1 to the first layer:

mc1._lockroot = true;
mc1.loadMovie("external.swf");
countdown = function(){
trace("ok");
}
countdownIt = setInterval(countdown,1000);

This code works fine in most of the cases, but I found many SWFs files that it dosen't work with. These SWFs and work with the following code instead:

loadMovieNum("external.swf",0);
countdown = function(){
trace("ok");
}
countdownIt = setInterval(countdown,1000);

I really try to use that code - BUT !!! -> if I use this code the external SWF predominant, my main SWF code and all the rest of my code dosen't work (like the setInterval() timer).... I want to load the SWF like the second example and still run the setInterval() function.