My 76 tach shows a steady 200 RPM too high, when compared to my meter. It's a Fluke 115, I use DC hertz to measure, then do some math. I've had my tach dash jump around and misbehave with a new ignition module,even though the engine ran fine. An extra condenser on the negative wire brought it back to life. The tachs seem sensitive to noise. They don't need much "signal" to work, there's actually a 2,300 ohm resistor inline with the coil, on the 76. Your 240Z tach uses the positive side though but may still be sensitive. Maybe a new condenser or an extra on the circuit would help damp some noise.