Looks like a light leak problem. Often the light is hitting the film between the supply or take-up spool and the film gate. As such, it's the frame before or after the actual picture that gets exposed and the band may go out to the edge of the film rather than being masked to the frame area. You might check for that. In my (thankfully) limited experience the problem may not appear on every frame because of variations in the amount of ambient light and/or the angle the light hits the camera. If the leak is very small, even the amount of time between shots makes a difference.