I would have to agree that loading the reel is a good place to look. I would also suggest examining the film to determine if there is a pattern to the dark or light areas. Do they get closer together or farther apart from one end of the film to the other? If so, check the bearings at the load end of the reel to ensure both move freely and do not bind. Plastic reels are more flexible (especially the ones that can be adjusted to multiple sizes), make sure you are not pressing it together when winding film onto the reel. If the areas of darkness and lightness are uniformly spaced, it would point to the camera.