Looking for an array guru here... Can you help?!

My project has about 30 movie clips in the library. With an action that comes up every few seconds, I'd like to randomly load one of those 30 clips onto the stage and have it play until it's done.

BUT - once a movie clip is already out, I don't want it to reset or come out again. I'd like the next action to randomly call out one of the remaining 29 clips.

Once each movie clip is done, I need some action to make it go back to the "available" list so that it can be called out again.

I'm guessing that I need to use attachMovie to call out the movie clips, and an array (maybe two) to make sure that I only get one instance of each movie clip. But after lots of searching and various dead ends, I'm just not finding exactly what is needed.

Can you help?? I'm on a very tight project timeline, so could really use some specific code.

Thank you!