Random starting point (motion tween)

First of all: I barely have any experience with Flash CS3, in fact I started two days ago. I don’t know anything about actionscript.

Im currently working on a banner for a client where bullets fall down on the screen. The bullet spins around via motion tweens. It looks pretty good and realistic, however the problem is that the animation only takes 3 seconds. In the third second there are no bullets on the screen so it’s very clear to the viewer that the animation restarts.

I was thinking about a random starting point for the motion tween to fix this but I have no idea how to acomplish this. Here’s an image to give you an idea what I want. The white is the area of the banner.

Help very much appreciated!