A couple of questions, hou1b:
1) What is the full version number of your copy of MediaLab? You can find it (i.e., v.20xx.x.xxx) by opening MediaLab and selecting Help>About MediaLab from the main menu bar.
2) When you say that it runs fine in one condition but not the other two, do you run the conditions in a particular order when this error happens? For example, is it a particular condition that always works or breaks or a case where whatever condition is run first works and everything that follows after doesn't work with the video? Try running a each condition by itself, closing MediaLab, reopening MediaLab, and then running a different condition.
3) Have you tried other types of movie files besides a .mov file?
4) Does the video file play find if you run it directly in Windows Media Player?
I'll also be happy to take a look at any files you send me. You can either upload them here or directly send them to me at support@empirisoft.com. If you send them to my email, please refer to case 16899 in the subject line.
Jason Reed
Empirisoft Software Support Specialist