I did tune-up and driveability for over 20 years within GM dealers. I had always known a tachometer can cause ignition problems but never had one cause any problems at work.
I'm sure you have figured out my 8 cylinder converted 914 unit was causing me problems. I had some misfire issues intermittently, always the best kind of failure. Well it started to really miss, brake down even at light throttle today. I decided to pull over and pull the plug, but I had the wiring sealed up tight and couldn't remove the wiring at the coil. The great thing about a 914 is you can pull the gauges right out. I disconnected the tack wire and it ran perfect.

Weird.

Coil or HEI?

I am running Edelbrock tune port injection, just like on the mid-late 80s Camaro and Corvette. It has a coil and a signal amp. I have seen a lot of tachometers react to a bad plug wire etc but this was different, it would gain rpm when the engine wasn't . Maybe there is a root cause still laying in wait like the signal amp which may have a week signal, at this point I don't know... I am going to connect another tachometer.