Tach stop after car warms up
 

I have a 85 rx7 that runs a little rich, but after the car warms up the tach and trailing timing stops completely.
After the car sets and cools off the tach starts working again with no problem.

To date, I have replaced the alternator, all relays under the hood, all relays and switches under the dash, both ignitors, distributor, wiring harness, and the acv valve.

The only other item that is strange is that the ECU box (that I replaced) continues to make a buzzing sound.

Yes, I have a lot of extra parts.