I'm not an expert, but see if this helps you.
What you need to do is to:

a) create a dynamic text box for each movie. Give it a variable name (open the
properties panel) and type box1,box2, box3, etc. for example.
b) in the first keyframe of each movie declare the variable
See that all the variables are declared at the beginning of each movie or
movies.

message1=[ "Welcome to Flash", "This is great!", "I'm learning a lot"];
message1=box1[0]; (it will display message "Welcome to Flash")

c) In the last keyframe of each movie to make the text disappear type this:

box1=" ";

Change the values as you change movies.
Do this for evey movie and test them again.

Good luck,

German