The speedometer gets the speed reading directly from the transmission. All A bodies most likely used a cable type system to connect the transmission to the speedo. From what I've learned, if the speedometer is bouncing around, the cable might need to be lubricated with new grease or the cable is bad, the cable shouldn't be too difficult to remove from the trans if that's the case. Less likely cause would be either the mechanism in the instrument cluster is bad or the plastic gear for the cable in the transmission is worn.
