as3 particle experiment funny business

[woodrobot]

Help... why do these movie clips hang around??
I was trying to create a general class for particles, but some of the particles hang around
for some unknown reason. Any suggestions on what could be happening?
here's the code from the fla:

stop();

var wide = 400;
var high = 300;
var timer =5;
var foglayer = new Sprite();
var blur = new BlurFilter(10,6);
addChild(foglayer);
foglayer.filters = [blur];
for (var i=0; i<100; i++) {
	addParticle();
}
function addParticle() {
	var ww = wide /2;
	var hh = high /2;
	// stars splashing up
	var particle = new Particle(Spark,this,ww,high);
	particle.xv = randRange(-5,5);
	particle.yv = randRange(-10,0);
	particle.clip.scaleX = randRange(.5,1);
	particle.clip.scaleY = particle.clip.scaleX;
	particle.fade = .98;
	particle.grav = .1;
	// smoke coming from mouse
	particle = new Particle(Cloud,foglayer,mouseX,mouseY);
	particle.xv = randRange(-1,0);
	particle.yv = -3;
	particle.shrink = 1.01;
	particle.spin = randRange(1,3);
	particle.fade = .99;
}
function randRange(min:Number,max:Number) {
	return Math.random() * (max - min) + min;
}
function Update(e) {
	addParticle();
	count.text = this.numChildren+"";	
}
addEventListener(Event.ENTER_FRAME,Update);

And here from the Particle class:

package {
	import flash.display.*;
	import flash.events.*;
	
	public class Particle extends MovieClip {
		public var clip:DisplayObject;
		public var wide:Number = 400;
		public var high:Number = 300;
		public var xv:Number = 0;
		public var yv:Number = 0;
		public var fade:Number = 1;
		public var drag:Number = 1;
		public var grav:Number = 0;
		public var xs:Number =0;
		public var ys:Number =0;
		public var oxv = 0;
		public var oyv =0;
		public var shrink = 1;
		public var spin = 0;
		public var clock =0;
		public function Particle ( symbol:Class, target:DisplayObjectContainer,xpos:Number, ypos:Number)  {
			clip = new symbol();
			target.addChild(clip);
			clip.x = xs = xpos;
			clip.y = ys = ypos;
			oxv = xv;
			oyv = yv;
			addEventListener(Event.ENTER_FRAME,update);
		}
		public function init(){
			clip.alpha = 1;
			clip.x = xs;
			clip.y = ys;
			xv = oxv;
			yv = oyv;
		}
		
		public function update(e) {
			clip.x += xv;
			clip.y += yv;
			xv *= drag;
			yv *= drag;
			yv += grav;
			clip.scaleX*=shrink;
			clip.scaleY*=shrink;
			clip.alpha *= fade;
			clip.rotation += spin;
			clock++;
			if (offScreen()||clip.alpha <.1 || clock > 1000){ kill() }
		}
		
		public function kill() {
			this.removeEventListener(Event.ENTER_FRAME,update);
			this.clip.parent.removeChild(clip);
			//this.parent.removeChild(this);
		}
		
		public function offScreen():Boolean {
		var xx = clip.x;
		var yy = clip.y;
		var ww = clip.width/2;
		var hh = clip.height/2;
		if (xx > wide + ww ||xx< -ww ||yy< -hh ||yy>high+hh) {
			return true;
		} else {
			return false;
		}
}
	}
}

I think it might have to do with the function kill() but I am not sure.