flash我在一段场景中运用action雪花飘落但是一直飘停步下来怎么使他停下来

在一段场景中要运用雪花飘落效果，后面不用，但是我的后面一直飘怎么让他停。代码如下this.onLoad = function (){
n = 60;
var i = 1;
while (n >= i){
this.attachMovie("snow", "snow" + i, i);
var a = Math.round(60 * Math.random() + 41);
var b = Math.round(50 * Math.random() + 51);
with (this["snow" + i]){
_x = 550 * Math.random();
_y = 400 * Math.random();
_xscale = a;
_yscale = a;
_alpha = b;
_rotation =a;
this["snow" + i].x = Math.cos(Math.PI * Math.random());//雪花沿x轴每帧播放后的位移增量
this["snow" + i].y = 2+ 2*Math.random();//雪花沿y轴每帧播放后的位移增量
}
i++;
}
}
this.onLoad();
this.onEnterFrame = function(){
var a = 1;
while (n>= a){
with (this["snow" + a]){
_x += x;
_y += y;
_rotation += y;
if (_y > 400){
_y =0;
}else if (_x>550){
_x=0;
}else if(_x<0){
_x=550;
}
}