Deano,
I am no MSD ignition expert by any means but if I understand correctly the tach worked ok until after you took apart the MSD set up to work on it. Then it started bouncing and then quit. That would sound more like a loose connection somewhere or a possible short. I use a Jacobs but I had that happen and the trigger lead connector had gotten loose over the years and it took me a while to find the connector in the loom. I now have a schematic of just where it and all other connectors are located. I am sure that you have already checked all of this but just thought I would give it a shot. Another way I have tested a tach is just take it and run wires from it to one of my other cars.
Ron
