Assuming the cap/rotor/leads and plugs are OK and gapped properly (try 0.6mm if they are appreciably bigger.. what plugs are you using?) then look into poor/intermittent connections:

1, check all connections to coil and ignition amplifier.
2, Check the condition of the three loom connectors on the bulkhead behind the engine, just above the cam cover.
3, Check/clean the loom earth tag connections on the inlet plenum cover - there are two earth tags with brown wires fitted under a couple of plenum chamber cover bolts, they must be clean and secure.

The tacho fluctuation (reading increases above where it should be, yeah?) is caused by interference getting onto the ignition amplifier trigger signal wire(s) - this is usually caused by poor condition HT components or bad earth connections to the ignition amplifer and associated loom.

Hope this helps.