Tachometer problems

So we have been working on this nifty Tecnam P2006T.   It's always sort of had some funny RPM indication issues, at first it was just the occasional indicated prop overspeed (flashes) but the pilot had both props synced and he's pretty reliable and trustworthy.

The Tecnam P2006T in question is equipped with two Rotax 912S that are dated 2019 G1000 type unit and RPM is indicated with prop RPM so I am just going to call it PRPM going forward here

Recently the problem became more aggressive; now during acceleration through about 1900 PRPM the tachometer will indicate redline and just show ---- with the indicator on the Garmin G1000 or G950 on three occasions showing yellow X on the right PRPM gauge.   Here's where it gets interesting, I can "fix" the problem by shutting off the right ignition switch (which I learned AFTER swapping out the Tachometer Trigger Coil), PRPM reads steady and accurate. 

Obviously this is not an airworthy fix

-Naturally I start checking for interference from the ignition leads by swapping the ignition module leads on the Coil side, problem is still neutralized by turning the right ignition lead off.

-I swap the connections back to their original position towards the coils and swap the connections on the airframe/trigger coil side of the ignition modules, problem is still neutralized by turning the right ignition lead off

-I swap the connectors back to all their original configurations so now left ignition module is A and right ignition module is B

-I swap the connectors on the ignition switches and now the problem has switched over to the left ignition switch

-Seemed to me at the time the issue is likely the P-lead must be feeding interference into my tachometer sensor.

-Replaced the entire length of right ignition switch P-lead wire right up to the airframe connector labeled P46J and installed a new ground for the shielding but the old wire really didn't look too bad, honestly it looked very clean and brand new.

-I also couldn't find the ground for the tachometer shielding in the same area so I just ran one closer to the main wire as it looked like the shielding probably grounded at the GEA71 unit in the back of the plane and this would be quicker to just get it done.

-I ran up the engine after all the wires were gone through as previously mentioned and threw in a Tanis because it's cold out here in Manitoba

-Problem has now emigrated to the left ignition switch.

I had looked pretty closely at the condition of the P-lead wires and didn't feel like changing the wire accomplished anything nor grounding the airframe side of the tachometer wire, so I guess I might need to be somehow more conscious about where these three wires go through the bundle as maybe having it all taken apart and put back together just meant that I moved the issue because of the tachometer wires proximity to my P-leads.

Any tips tricks or advice would be always appreciated but for now my next move is to fully isolate the tach wire all the way from the engine to the GEA in the tail which means I need to de-bundle and re-bundle this all again