So what were the culprits denying me this victory? It turned out there were several:
- The stock wiring harness has shielding added for wires that connect from the PCM to the Distributorless Ignition System. On my harness, some of this was missing. In addition, I extended two of these sections, initially without shielding, because I relocated where the DIS was mounted. It was necessary to go back and rebuild this portion and add the shielding.
- The main ground connection from the PCM passes through a single-pin connector. On previous testing, this ground circuit tested fine. Now it wasn't. The culprit turned out to be the pin on the male side of the connector was making intermittent contact. By cleaning & widening the end of the pin I was able to get a reliable ground connection. A new connector will be secured for the final installation.
- When the motor was originally rebuilt I replaced the coolant temperature sensor with a new one. So this was one of the last sensors I got to when trying to resolve the running issues. It turned out that one of the two pins in the sensor failed - probably the last time I assembled the motor and reconnected the sensor to the harness. Fortunately I kept the original sensor which tested fine.
So at long last the motor runs well. It will still need hours of bench running so the new rings can seat and the computer can relearn its configuration settings, but it purrs like a kitten and roars like a lion.