Erratic rpms Rotax 912/Garmin G3X

Here's the custom board I bought from an aviation company I think in Virginia.

Here's the premade circuit board I bought from an aviation company I think from Virginia.

And by the way, I just tried installing FOUR ferrite connectors, all along the rotax tach line (up to firewall), and it made zero difference. 

Ferrites will only have an effect on very high frequencies (MHz) the problem is more likely much lower down the spectrum in the lower kHz region. You would need to 'scope the waveform to see the problem so you know what you are up against and then you would stand some chance of fixing it. Everything else would just be trial and error.