Referencing Duplicated Movie Clip In Separate Code Block

programming flash
1

I have the following code:

bubble_mc._visible = false;
var bubbleCount:Number = 40;
var vertMaxSpeed:Number = 10; // will be used in calculating maxiumum vertical speed for bubbles' movement

dupMovie =  function ()  {
	for (i=0; i<bubbleCount; i++) {
		bubble_mc.duplicateMovieClip("bubble"+i+"_mc", i, {_x:Math.random()*bubble_mc._x, _y:Math.random()*bubble_mc._y, _visible:true});
		with (eval("bubble"+i+"_mc")) {
			_height = _width = Math.random()*bubble_mc._height;
		}
	}
};
dupMovie();

onEnterFrame = function() {
	for(i=0; i<bubbleCount; i++) {
		var randomSpeed:Number = Math.random() * vertMaxSpeed;
		"bubble"+i+"_mc"._y += randomSpeed;
		if ("bubble"+i+"_mc"._y < 28) {
			delete "bubble"+i+"_mc";
			bubble_mc.duplicateMovieClip("bubble"+i+"_mc", i, {_x:Math.random()*bubble_mc._x, _y:Math.random()*bubble_mc._y, _visible:true});
			with (eval("bubble"+i+"_mc")) {
				_height = _width = Math.random()*bubble_mc._height;
			}
		}
	}
}

The problem is that Flash whines when I call

"bubble"+i+"_mc"._y += randomSpeed;

Because it won’t let me reference the duplicated movie clips that way… is there any other way to do this?