I was having an issue with my Smiths Speedometer where it would flicker/jump around and not maintain a constant value at a constant speed.  Since the speedometer is mechanical there are only a few things that could be the cause.  The angle drive which turns the internal transmission rotation into external gear rotation via an angle drive attached to the upper-right side of the transmission.  For my problem I did not replace or fix the angle drive.  The next part that can be the problem is the speedometer cable which attaches the angle drive to the back of the speedometer.  This part is known to fail if over lubricated, which causes some of the excess lubricant to enter the back of the speedometer and gum up the internal workings.  The speedometer cable can break or become too dry and stick or more often the square-head ends can become rounded causing them to slip intermittently and not give an accurate reading or a jumpy reading.  Third, the internal gearing inside the speedometer can be faulty which will also cause erroneous readings.  I hope you enjoy the video and there are extra pictures linked below.