Welcome to APUG.

Does the dust/squiggle remain EXACTLY at the same spot and in exactly the same shape and orientation? Is it sharply defined? About where in the frame is it? Middle or at the edge? Can you scan the image and post it?

Your camera has a focal plane shutter. You can think of it as a SLIT is moving across right in front of the film. I wonder if it is sticking or dragging at high speeds? If you open the back and remove the insert, then trigger the shutter, you should be able to see this. Is the orientation of the dark band same as the direction that this slit travels?