con una cosa del genere

posX=ball_mc._x;
var SPEED:Number = 5;
var center:Number= Stage.width/2;
var rightLimit:Number = Stage.width-ball_mc._width/2;
ball_mc._x= center;
dove=true;
ball_mc.onEnterFrame = function(){
if(dove){
this._x += SPEED;
}else{
this._x -= SPEED;
}
if (this._x>rightLimit) {
//this._x = rightLimit;
dove=false;
}
if (this._x<posX) {
//this._x = rightLimit;
dove=true;
}
}