Check the short piece of wire going into the distributor.
I had a similar thing happen to me with my factory 120y coupe tacho.
The tacho needle was jumping around quite heavily, and the car wasn't starting all the time.
I tried another coil, but it still did the same thing, so I was checking the distributor and came across the broken wire inside the distributor.
So I replaced the wire and it fixed the problem straight away.