I got this effect from a very good French Flasher. I found the code on his site, flasheur.com , broke it down, and improved it a little bit, all this just for you.
You'll find that in this tutorial, I also took the code from the Random motion tutorial, that I use the swapDepths and duplication functions, and a lot of actionscript-based motion.
So that's not an easy tutorial, and you'd better check the other tutorials before attempting this one.