BoxeDK

I bought an aftermarket tacho because I don't have a stock one in my truck.
The car is a 1992 C1500 with L05 350 engine and 700R4.

Simple wiring to tacho:
Red to 12V ignition.
Black to ground.
Green to negative site of coil. (That would be White in my case)

Hooked it all up, and started the engine. All looks good. Also when reving...

But as soon as I put the vehicle in gear and drive, the tacho doesn't show more than about 3000-3200rpm... Just bounces between there...
There is no problems below 3000rpm... Maybe a little wobling tach-needle but not much...

The problem only shows when car is moving. Not if it stand stills in neutral and I rev the engine...

I've tried another tacho.
I've tried the "10K-resist-in-tacho-signal-wire" trick
I've tried another ground wire to a different place.

Nothing solves the problem...

Anyone knows whats wrong?