Movie freeze on site re-visit in Firefox and Opera

Hi all,
I'm very new to flash development. I've done a movie that is about 1.5MB in
size. I created a pre-loader that just flips from frame 1 to 2 until the
percentage complete is greater than 99%.
On Internet Explorer 7 the movie works fine every time. If you've visited the
site before the pre-loader is skipped and the movie starts from frame 3.

With Firefox and Opera something strange happens. The first time you visit the
site it all works perfectly. When you go back to the website without clearing
the internet cache, the pre-loader doesn't work it just stops at a given
percentage and then a pop-up says a script is causing flash 8 to run slowly.

Does anyone know what causes this? I've seen posts that recommend putting a
few extra frames in and adding a hi-res image to them to give the pre-loader
something to do, but this doens't seem to work. It just increases the movie
size!

Any help what-so-ever would be very much appreciated.
Thanks