This is going back to my film days. Assuming that the shutters work by the same principle, At low shutter speeds, the first shutter opens and the frame is completely open until the second shutter closes. At some point -- and this is how my eye perceived it -- the perception for high speeds is that the gap was never fully open. It appeared to the eye as if an open bar bordered by the trailing edge of the first shutter and leading edge of the second shutter traveled across the frame. The higher the shutter speed, the narrower the bar. I get the feeling that in your case if the first shutter is dragging something it would have more of an impact at higher shutter speeds. This is speculation on my part. In any event it sounds like it's time for a trip to the repair shop.