A very distinctive symptom. A sticking shutter curtain which allows more light to strike the film at a certain point in its travel. When you press the shutter release the first curtain begins to open. Shortly after that the second curtain is released and begins to follow the first. Other than a short time parallax between the two motions the travel of the two curtains must be at the same speed. When the second curtain does not move smoothly (sticks) you get the distinctive exposure pattern seen in the sample. If the problem should be reversed and the first curtain sticks then you would see a band of reduced exposure.
Check to see that the grooves that the curtains run in are clean and there is no foreign material there. Another possibility is dirt in the gear train that controls the shutter.