Velocity is on the right track..
It is caused by interference on the ignition amplifier inputs, generally due to poor earth connections from the ignition amplifier/coil circuits.

1, Check condition and security of all connections to the coil, ignition amplifier, crank and phase sensors (on the side of the distributor)

2, Find the earth terminals on the inlet plenum chamber that bolt down the brown ground wire terminations from the loom - dismantle if neccessary to clean and refit.

3, Similarly ensure the cleanliness and security of the ground connection on top of the bulkhead just in front of the battery.

4, If it's a 4x4 - check cleanliness and security of the three loom connectors on the bulkhead above the back of the camcover.

you will be unlucky if this doesn't sort the problem. The jumpy tacho thing is the first indication of this problem.. if left to deteriorate, it will often result in misfire as well.