1986 grand am, 86,000, 2.5 FI, I have changed the instrument cluster and the speed spensor, my speedometer still jumps and only works periodically but usually works after about 3 miles, or when car is warmed up. Can you tell me what the next step would be to check. Could it be electrical? If it is what wire do I trace? Any suggustions would be greatly appreciated. Thank you.