Thanks for the update on what you have done Nat. Was the gear wheel part stripped?
I have experienced similar problems. I assume you are sure the signal from the diff is 100% all the time.
Reflowing the the speedo pcb and testing the caps and resistors values can be a fix but it can also be the chip on the pcb is playing up after all these years. I have even found that the tightness of the 2 screws on the stepping motor can be problematic. I regret to say have had to admit defeat on some speedos in the past!
Sorry this isn't much help!
Bob
