There was a solid clue during setup...when sliding the connectors into the harness...they are whisper-tight. Meaning: Not at all. Solid money is on worn out/loose harness connectors but we'll see if banging on ECU does anything after the normal "give up and die" timeframes of 20-40 mins.

RESULT: Loose harness connector(s). Now to figure out how to either tighten/tweak the harness connectors...or replace them 🫤

There is still a mystery (to me, at any rate) on why the engine would sometimes simply shut off like a switch and sometimes stutter-stop, and why sometimes it would re-start immediately and othertimes re-start 10-20 mins later. My hypothesis is that by simply getting in/out of the car while diagnosing I was re-wiggling the loose connector(s) such that they had good-enough connections, and the changing fault symptoms were a result of different pins or a different combo of pins losing contact.

The end result is the same: Fix all the connections and do pin-drag tests on all of them to make sure they are sufficiently grabby.

Thank you to everyone. I hope it was fun for you too.

Don't overlook that when you have the connector disconnected from the board that the solder joints are probably less-stressed. I didn't see the common cracked solder joint problem mentioned in the thread.

For some reason the internet is not picking up the old threads when I search. I only came across one. I've seen the others though.

2 hours ago, f1d094 said:

RESULT: Loose harness connector(s). Now to figure out how to either tighten/tweak the harness connectors...or replace them

Glad you found something. Are you sure you have identified the root issue there? Reproducible at will? Double dog sure?

And as for the mystery about behavior, electrical intermittent connections are like that. Barely making connection by a molecule. Does unpredictable stuff.

Oh, and it's been many moons since I've looked at that connector on the end of the EFI harness. I assume the terminal contacts inside are replaceable? Have you looked into that?

For the record, I just read that zcar.com thread and see that I might have actually borrowed the transistor replacement idea from ZXPastor. Not sure. I got the replacement transistors at a Fry's Electronics store before they shut down.

Cracked solder joints aren't uncommon on the old Datsun electricals. The headlight and running light connections, on the top of the steering column, crack and break also. You might add some relays before those fail. Full lighting circuit current runs through them. They get hot.

Will be following to see how things work out. You'll have three ECUs to swap in and out for testing and comparison. Good times.