Disappearing Movie Clip - Why?

I have a slideshow that loads slides in using the attachMovie() to put movie clips into a droptarget, the transitions fade in the next slide with actionscript. However, after an extended period of time, the slide just disappears and the then throws off the other arrays in the slideshow. What on earth would be causing this?

Here’s the code for the transition:
// See if the aDest has been changed
if (this._alpha != setAlpha) {
this._alpha += (setAlpha)*.05;

I’m not sure if that’s it, but if I slow the timing of the actionscript tween, the longer the slide will last on the screen. It seem to work fine when the tween is set to .01, but anything higher, the slide will disappear (or perhaps remove itself) after about a minute.