QuickTime 7 error: changes duration when performing Add and Scale

Question

Sometimes, when using "Edit - Add to Selection & Scale" to place an image over a portion of a movie, QuickTime Player 7 will incorrectly change the duration of the movie, sometimes increasing the time by HOURS!
This may be an old problem, but I couldn't find any mention of it in the past year on the forum.
To make it easy to reproduce, I created the following AppleScript. Usually I can only get 5 to 10 iterations before failure.
(* Demonstrate QuickTime bug
This script demonstrates an error that QuickTime Player 7 commits when adding an image to a movie.
(This error also occurs when performing this operation manually, but it's faster to use AppleScript to demonstrate it.)
R. Lougheed, December 2010
set maxTestCount to 90 -- QuickTime has a limit of 99 tracks
set sampleMovie to (choose file with prompt "Select a movie" default location (path to movies folder) without invisibles)
tell application "QuickTime Player 7"
activate
close every document saving ask
open sampleMovie
set movieRef to (a reference to document 1)
-- get a frame to paste in
tell movieRef
set movieDuration to duration
tell me to set midTime to round (movieDuration / 2)
set current time to midTime
copy
end tell
end tell
set inTime to round (movieDuration / 3)
set outTime to 2 * inTime
set successCount to 0
set newMovieDuration to movieDuration
repeat until ((newMovieDuration ≠ movieDuration) or (successCount > maxTestCount))
with timeout of (1 * hours) seconds -- default timeout is 2 minutes
tell application "QuickTime Player 7"
activate
tell movieRef
select at inTime to outTime
add with scaled -- this should NOT change the duration of the movie
set newMovieDuration to duration
if newMovieDuration = movieDuration then set successCount to successCount + 1
end tell
end tell
end timeout
end repeat
activate
if newMovieDuration = movieDuration then
display dialog ("QuickTime Player successfully ran " & successCount & " Add-with-scale operations.") buttons {"Great!"} default button 1
else
display dialog ("QuickTime Player erroneously changed the movie duration after " & successCount & " successful operations.") buttons {"Bummer"} default button 1
end if

Answer 1

Thank you very much Hiroto !! But I've still some issue that I can't resolve by my self...
I don't understand how to take this script (who works very well) and to use it in a different file. Actually, I wanna ask this script by 6 different other applescript file with a different movie (one in English, one in French, one in German, one in Dutch, one in Spanish and one in Italian). So I try an applescript like this :
set FR to LancerFilm()
tell FR
quitQT()
set x to "/Desktop/Robin.des.bois.2010.DVDrip.VF.XVid.avi"
setFilm(x)
startFilm()
end tell
I tried this code at the end of the file who contains the main script and in an other file. For the first one, I receive an error message who says me : +Can't find the file /Desktop/Robin.des.bois.2010.DVDrip.VF.XVid.avi+. And for the second one (wich is the solution that I wanna have for the final script), the system says me : +<<scrip>> doesn't understand the message LancerFilm+.
If somebody can help me it will be awesome.
I have an other question. At the end of the movie what must I do to launch another movie, automatically (so in the same script) in loop ?
PS: Sorry for my pour English