Basic anim slightly jerky

Stick this in a frame on a new document at 30fps:

MovieClip.prototype.scaleIt=function(me){
if (me._xscale < 300) {
	 me._xscale = me._yscale += 1;
	}else{
	 me.clearInterval(this.intrvl);
}
};
MovieClip.prototype.startScale=function(){
this.intrvl=setInterval(this.scaleIt,5,this);
 
};
this.createEmptyMovieClip('box',1);
box.beginFill(0xe000000,100);
box.moveTo(50,50);
box.lineTo(50,100);
box.lineTo(100,100);
box.lineTo(100,50);
box.lineTo(50,50);
box.endFill();
box.startScale();

Could somebody explain to me why it is jerky? I just dont understand why the anim is all jittery…esp on the left/right side (top/bottom seem fine)… Moving it faster makes it worse as well.

Many Thanks