Setup.... stock 90 DX hatch, swapped in a USDM cluster with tach. Tach bounces while driving will jump all over from 3000 to 6000 down to 4k then 7k etc... completely random. Distributor is good, new igniter, coil,cap rotor wires. Engine grounds have been checked and re-checked, ECU fine. Had an Autometer tach in for a couple months and it did the same thing. Any ideas would help, also no other problems in the dash with the heater, radio, speedo, lights etc. Borrowed a different cluster thinking there was something wrong with the one I had and it did the same thing.

 

Interference in the wiring, bad igniter, or bad wiring (shorting out).