If the shutter curtains are leaking light, then it's not just the action of the curtains moving across the film, but also the film moving past the curtains when being advanced that causes the striation. So if the deterioration is bad enough, you're likely getting the leak while rewinding the film too, adding to the streaking, and thus the in-between frame exposure. And nearly twenty years on shutter curtains of a camera that may not have been stored under the best environmental controls could easily have worn them out, depending on the quality of material used. I've read that a Leica overhaul is only good for about ten years, no matter who does it.
Waiting for the light