Those garbled lines are the result of a corrupt file, meaning the problem is inside the .mp4 file and has nothing to do with the software you're using.
Are those the original files ? If yes, your only solution will be to use the services of a data recovery company like this one : http://mp4repair.org/pwt6/preview.html
If those were copied / acquired / transcoded from another media which you still have and which does not show signs of corruption when played back, try copying / acquiring / transcoding them again.