swf file disappear from the screen

We have a problem with a code as below made in AS2. By some reason 3-4 minutes after appearance the image disappear from the screen. Please, advise, where is the error?

var picTotal:Number = 1;
var lightSpeed:Number = 5;
var delay:Number = 5000;
var count:Number = 0;
function init():Void {
var white:MovieClip = this.createEmptyMovieClip("wh", 10);
with (white) {
beginFill(0xFFFFFF);
lineTo(Stage.width, 0);
lineTo(Stage.width, Stage.height);
lineTo(0, Stage.height);
lineTo(0, 0);
endFill();
}
white.blendMode = "add";
lightSpeed = -lightSpeed;
showNext(white);
}
function showNext(clip:MovieClip):Void {
clip.onEnterFrame = function():Void {
this._alpha += lightSpeed;
};
}
init();