I created this demo file that you can look at. I don't have time right now to explain how it works, but I have put a lot of comments in the code so that should help you. In the main timeline, click on the textMovie clip and look at its Object Actions (either right-click and choose Actions, or click on the blue arrow in the Instance Panel) -- everything happens there.

I should get time this lunchtime to explain it a bit more.