As you get more used to FLASH action script its cool to make an mc do its own thing, that leaves the the main timeline open to whatever. So when you double click on the particular mc to edit it, when its surrounded with a blue box, you are within the mc's "action". Try placing an

onClipEvent(enterFrame){
if (this._x <= 400){
return;
}else
this._x += 2
}

there, and then the mc does what its supposed to, and leaves you to utilise the main timeline even more.

swills