It may be manufacturing defect or scratched disk, even if you do not believe in this. I guess you are talking on new pressed DVD-Video.
The fact that the error is not at the same place just means that the defect/scratch is longer than you think and so has marginal problems at diferent locations.
Try cleaning the disc with the toothpaste trick, where you get some toothpaste on clean wet hand and first dissolve it, then clean the disc, let it dry, and clean dry. Remeber all motions must be radially from the center out. Search forums for better description on this method of cleaning.
If this is disc you bought, exchange it with new one.