I had a noise and a bounce with the old speedo cable so i replaced the speedo cable and now the noise is gone which is great cuz it was very annoying but now it bounces worse then it ever did.
My dad and I are redoing a recently acquired '91 GT. The speedo on it would jump at low speeds and go to 120 - 140 at anything over about 45MPH. We found a premo complete gauge cluster on ebay with only 25K miles on it for $100. We also lubed the cable. Problem fixed.