I had the same problem when I had TPI with an MSD ignition. My problem... the coil wire was too close to the white tach wire. I'm sure high energy in the wire was somehow bleeding over to the tach wire making it jump all over the place. You could try relocating the coil too.
Another thing that was happening... the hood was rubbing on the top two wires going into the coil (if you still have it mounted in the stock location). Eventually from engine movement it rubbed the plastic jacket off the wires and it would make them ground out effectively killing my engine or makit it miss a lot. Another thing to check I guess