If the movie plays all the way through in the iMovie project timeline then the 8mm import completed correctly. So your issue pertains to a faulty export, unless there is some glitch in the playback app. Sometimes export issues can be caused by a corrupt iMovie preference. Try deleting preferences and then see if you can export and play the entire movie.
To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
-- Rich