Your cluster has an open (that is break) or corrosion in one of the connections or else on the sensor wire into the cluster. You can clean up the connections on the back of the cluster under the screws by lightly using a pencil eraser which might solve it. The twitch you see is a sneak path for the current surge from the headlights into the speedometer circuit associated with the open circuit. If you are sparky friendly, you can fix it, otherwise a repair or replacement.