Originally Posted by Stu @ M Developments
Bad connection in the plugs on the firewall are a 90% probability pal.
Failing that, run a new earth lead from plenum to battery and then battery to ignition amplifier.
4x4?
The clue is in the mental dancing tacho
no harm checking the crank and phase sensors as suggested, but my 90% is on this too. 85% of it is on the multiplug nearest the battery with the thick wires in (we're talking about the three multiplugs on the bulkhead above the back of the engine) - split the connector and check that the pins are clean, then plug it in and out a few times. Do the other two as well. Also check the condition of the earth tag connections on the brown wires that bolt onto the plenum chamber, and if you can as Stu says make up a stout earth lead and run directly from battery negative to the point where the brown earth wires bolt on the plenum.
Also double check the connections to the ignition amplifier and coil.