Disappearing MovieClip with _alpha code

Just checking if anyone else experiences this problem.

Create a movieclip, set alpha to 0, then actionscipt it with

onClipEvent (enterFrame) {
this._alpha += 10;
}

when the movie loads, its fine. Wait about a min or two, or leave it open in a window and do your other stuff :P, when you come back later, the movie has disappeared off the screen, either that or it comes on and off in about 1-2 min intervals.

Any ideas? Just wondering if it can be prevented.