Instrument Voltage Regulator

To narrow down the speedo issue. Get a friend to sit in the car. Remove the plug the cable is connected to at the trans. it has index markings on it. Spin the gear by hand, like snaping fingers. Ask the person is the speedo moved. If not spin the other way just to make sure you spun it the correct way. It the speedo jums a little the problem is not the gauge or cable. Maybe the plastic gear or trans. just a thought