From what I understand, you suspect that the film has a problem with perforation.
To check perforation of film the processing labs use of contraction ruler.
Variations of contraction ± 0.3 are supported.
Variations of contraction over 0.4 - 0.5 are critical.
If you do not have such a tool, take a film that you have not had problems over the film perforations problem overlap.
If you see perforations is shifted, then it is a problem with the film suspect.
Step perforation of suspect film is different.
https://en.wikipedia.org/wiki/Film_perforations
George
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