A light seal leak is more likely than a sticking shutter. The uneven edge of the bright band suggests stray light entering the camera, whereas a shutter fault would usually create a straighter, more uniform edge.

The most likely area is the camera back seal on the take-up spool side. Leaks there can appear in slightly different positions from frame to frame because the film winds onto the take-up spool, changing the film’s position and curvature. That also fits an intermittent problem that shows up more on longer exposures.

A good test is to place a small light inside the camera body, close the back, then inspect the outside in a very dark room after your eyes adjust. Any place light escapes is a likely leak point. You can also test from the front with the shutter open on bulb, or inspect the shutter from the film side with the back open to rule out shutter pinholes.

So: first suspect deteriorated back-door seals, especially along the hinge or latch/take-up side; shutter sticking seems less likely.