I don't know if over agitating would cause these marks, I would think maybe too little agitation might cause uneven development. I work in a photo lab and when I see marks as they have been described, they are what are known as stress marks. This is caused by rewinding film backwards into the canister, as the film kinks inside the canister it creates a stress mark that runs across the film from sprocket hole to sprocket hole. This may not be your problem or maybe it is....(posted 9602 days ago)