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.

maybe a problem with the tach?

Hey guys, I have put in hundereds of tachos in my time but this one seems strange....

The needle wont sit at the perfect revs, the engine is running smooth but the needle jumps from the idle (approx 800rpm) to about 1200 - 1300. This behaviour continues all the way up the rev range. I changed the coil to a new one, thought that might fix it but no luck. At one stage it did work fine, for about 10 minutes but then it went back to its old tricks. I have made sure it is on 4cyl and even got a replacement sent out, so it must be the datto!!!!!

Dont think it really matters but its a 81 Sunny with a A12.

Thanks guys/ gals