My 1200 tacho also works fine with the management I'm using, which also controls the solenoid which supplies power to the positive of the coil. It only starts going haywire when it hits the limiter, which cuts the positive to the coil, it will then jumps back and forth between 7k and change and 0.

So yes, have a look at your connections again, it sounds like the behavior stems from a bad connection.