"I have two AMMs backups that are OEM vintage originals (no refurb stickers) of unknown quality, tho one of them at least seemed to run fine in my car last year for a day or two. Neither of them have I properly tested with the multimeter in the test module of the ECU. "

With the tools we have at our disposal, Art B has pretty much cleared up the myth of the multimeter test on the AMM. Apparently it can identify a fully failed unit but little else. The O2 sensor test is still telling (does it swing .1-.9V)? Back to the AMM, the plug itself has been the subject of many a forum post. The pins can push back in the plug and make poor/intermittent contact.

I cannot remember if you have checked the wires cap and rotor. I again think these are unlikely suspect but a "stumble" for the LH2.4 cars, which I am more familiar with, are often linked to marginal ignition components and more reports of bad Bosch rotors and caps have popped up in the last 9 months than I have ever seen on this board. Also, you car is from the era of a bad distributor plugs...but you would have to do a search on that as out of my comfort zone.

Finally, the fuel pump relay on most 240's can cause similar issues but I think your year is virtually immune to the issue because I think it uses standard "ice cube" relays instead of the needlessly complex relays in the later and earlier cars. This is of course going on memory from an era of car I have never owned or worked on.

Hopefully someone who understands the unique feature of the LH2.2 will chime in. I have a giant hole in my experience that includes anything between K-jet and LH2.4.