Speedometer Goes Crazy Any Advice?

HI! MY 1989 SC / 5 SPEED HAVE A STRANGE SICK, i START UP THE CAR AND THE SPEEDOMETER STAR TO JUMP LIKE RECEIVING ELECTRIC PULSE WITHOUT IDLE OR RUN THE CAR, I KNOW MY BALANCER IS LOOSE , BECAUSE I FOUND THAT WENT I CHANGE MY CRANK SENSOR, BUT THE CAR CAR NOW START GOOD, I'M NOT USING THE CAR UNTIL I RESOLVE OR CHANGE MY BALANCER BUT WENT I WARM THE CAR I NOTICE THAT MOVEMENT IN THE SPEEDOMETER ANY ADVICE OR EXPIRIENCE TO SHARE?​