Subaru Tachometer Drive

I have a 2001 GC8 Subaru with an M130 running a EJ257 engine and have an issue with the tachometer output drive which I haven't been able to fully understand or resolve.

When setting the resource and simulating the output using the test function (without the engine running) the tachometer in the vehicle displays the correct RPM (with the scale set to 2Hz as the original engine used a wasted spark setup) across it's entire range.

Once the engine starts the needle flickers erratically a couple of times and then returns to the rest position. Occasionally whilst driving the needle will have one or two erratic movements. For the past 2-3000km I have left the tachometer wired and configured and it seems like there is increasingly more activity and on occasion the needle oscillates around the correct engine speed.

Has anyone had a similar issue with a Subaru?

I have not seen this before on Subaru but have seen it before on other cars. Most tach's need a 1k ohm pull up resistor. I can imagine that when the ECU is sending the function with motor non running there is mush lower interference on the output signal. Once the engine is running the signal becomes saturated with noise. By using a pull up resistor the signal level will be raised and will be easier for the tach to pick up the correct signal.