My tach keeps jumping but the engine stays steady.... Does anyone know why this is?

 

check the earth on the thermostate
also caused by the coil on its way out

 

Its your ignitor in your distributor, it sends the signal to your tach. Has nothing to do with thermostat ground or coil

 

This, the g10 ground could be a cause, but it is very unlikely. It sounds like intermittent contact is being made at the ignitor. Pull off your distributor cap, taking off the rotor is optional but it will make it a lot easier. There are three wires on the ignitor, clean the terminals with a fine sandpaper, or put some sort of conductive grease of the terminals.

 

that worked like a dream thanks.... didnt want to have to buy a new one they are hard to find in germany and very pricey