I'm using the following script on my flash movies to load swf movies into
levels:

but1_btn.onRelease = function() {
loadMovieNum("letterB.swf", 1);
};
unbut1_btn.onRelease = function() {
unloadMovieNum(1);
};
stop();

This script works fine on the first movie load but on the second movie if I
add the above code it goes to the new movie letterB.swf but it briefly shows
the first movie before getting there.

Why is this and how can this be avoided?

Thanks

you mean you're executing loadMovieNum("letterB.swf", 1); twice in a row
without loading something else into _level1 or executing unloadMovieNum(1)?

if so, you're going to see a flash of stage between the first unload and the
second load.

On my first .fla I have used

loadAButton_btn.onRelease = function() {
loadMovieNum("letterA.swf", 1);
};

On my second .fla I have used

but1_btn.onRelease = function() {
loadMovieNum("letterB.swf", 1);
};
unbut1_btn.onRelease = function() {
unloadMovieNum(0);
};
stop();

My thrid .fla doesn't have any script other than a stop action.

Does this help? I'm using MX2004

does it help what? if you're trying to explain your set-up, you should
probably explain the relationship among your first, second and third flas and
what causes your problem and what problem you encounter