onEnterFrame cpu

My swf works nice, only my it keeps increasing cpu.
Can anyone tell me how I can stop that?
I thinks its because of the onEnterFrame, but if I change that to onLoad or something, it dont work anymore.
blow is my code.

input_var._visible = false;
Stage.scaleMode = "noScale";
fscommand("showmenu", "false");
loadVariables("random.php", this);
onEnterFrame = function () {
    aantalLetters = input_var.length;
    for (i=0; i<aantalLetters; i++) {
        iets = random(8);
        beweegletter.duplicateMovieClip("beweegletter"+i, this.getNextHighestDepth(), {_x:i*25, _y:iets});
        _root["beweegletter"+i].tekst.text = input_var.charAt(0+i);