createEmptyMovieClip

i have a thumbnail gallery. all the thumbs are in an MC that i can slide to the sides. each thumb opens a big picture in a floating MC that has another empty MC that holds a big picture. i am trying to make a dynamic clip to hold the big picture so i can remove it each time because right now after awhilt the fllash starts to work very slow…
my problem is that i dont understand why i cant create an empty movieclip inside my holder… here is the code(overlay is the name of my holder movieclip)

overLay._visible = true;
overLay.createEmptyMovieClip("bigHolder", overLay.getNextHighestDepth());
overLay.bigHolder.loadMovie(Items[m][2]);
overLay.createTextField("text3", this.getNextHighestDepth(), 16, 314, 380, 102);
overLay.text3.multiline = true;
var my_fmt:TextFormat = new TextFormat();
my_fmt.font = "Verdana";
my_fmt.size = 11;
overLay.text3.text = Items[m][4];
overLay.text3.setTextFormat(my_fmt);

and here is a link to what the gallary looks like:http://www.tamisivan.com/temp/amiDov/amiDovOp3_3.html