My Tach starts to fluctuate when it hits 4500 rpm. Below that it is steady, but once it hits 4500 rpm it fluctuates to 5000 rpm. This is a new Tach, my old one didn't do this. This is a AutoMeter Marine Tach. I double checked all the wiring. Even ran a separate ground with no avail.

I take it Im on my own on this

I just had one doing it over 4000 and it turned out to be the distributor, play in the shaft was causing the pole piece to give varying signal at high rpm. New dist and it's rock solid to 5000. 96 502 EFI with Gaffrig tach.

Thanks, I'll check it out.