Shakey Tacho (not Standard)

[Richie-Van-GTi 71 2 Cars]

Bit of an odd one for you, in my project car I have a full set of autometer gauges, the tacho is hooked up to coil negative and it does read engine speed as expected. Problem is aith engine idling around 1k it reads around 700rpm and it shakes about all over, it seems to smooth out when the engines revved and if I bounce against the limiter it does seem right at the top end (reads about 6500 max on bounce), The engine seems to idle too well for it to be not firing all the time at idle and all the plugs are new. Any suggestions what it could be? Ive ran a resistance test on the wiring and theres no breaks in the wires between the gauge and coil and its not earthing off anywhere etc.

[moonas 0 1 Cars]

Are you using a shielded cable for tacho signal?

[Richie-Van-GTi 71 2 Cars]

I assume so, its what came with them.