The rain finally stopped so I took the car on a test drive with OE ignition ECU. It ran good. Next I plugged in my spare GM ECU. It ran good. Last I removed the two ground wires attached to the metal mounting plate and plugged in the mounted GM ECU and it also ran good.
The OEM harness runs two ground wires into the ignition ECU. They are crimped to one pin in the ignition connector. One of those ground wires goes to a ring terminal and is normally screwed down on the idle ECU mounting screw. Both the OEM and my GM ignition ECUs do run ground to the ignition ECU mounting plate. So it's not good for electrical noise to have two ground connections like that. Ring terminal ground on the lambda ECU also makes a third ground to the metal plate. The lambda ECU has no connection to the metal box it mounts in.
So I remember some owners having problems using the GM ignition modules and I bet those needless grounds were causing problems. You can test just by removing those two black wire ring terminals from the metal plate (let them float).
So my metal mounting plate is grounded at one point by the mounted ignition ECU. The OEM harness runs a ground wire to one connector pin and the pickup ground (shielded wire) goes to another pin but the ECU connects those two signals.