I have a Autometer set of electric gauges in my 34 coupe.  The speedometer works, but the needle jumps every once in a while.  I tried to isolate the sender wire from the tach wire to the coil and that helped some but it still jumps around some.  Any suggestions?  

If the sender is working off of a gear drive on the trans, the problem may be the gears are going bad. These are plastic if GM or the driven is plastic on Fords and Chrys.Removing the driven gear should show the problem. Check for apple coring of either the drive or driven gear and for looseness of either on the shaft or housing.Also check the connectors for corrosion while you're down there. RFI (Radio Frequency Interference)can be an issue if the sender wires are too close to high energy wiring but that problem is usually more erratic and sustained.The twist in the sender wires usually takes care of most of that.