The J4 connector is the only connector needed to boot the MPU. I have no idea on the Alltek boards though. I have only struggled through the original MPUs.

Congrats on keeping such a great classic title on route, it is a favorite in my collection.

My guess, the same as others, is that if two MPUs both keep resetting it is probably a bad/marginal connector.

I know they were rebuilt once, but if you can't find any other issue I would repeat the rebuild, perhaps a marginal connection at one of the crimps. I have had test points have the correct voltage, but that is only part of the story, a bad connection might not be letting enough amps through because of a marginal crimp or one where the wires have corrosion on them, thus the reset.

I know many think it is bad form, but I have soldered the molex connectors onto the wire after crimping them for critical areas like these, that way I am certain the wire to connector issue is solid. Also, I know if the wires are clean enough to wet with solder I know the wire was clean enough to have good contact with the connector. Some of the old wire in these harnesses has such bad corrosion for the first inch or two from the end of the wire that good conductivity is not possible without diligent scraping/sanding of the wire.

Best of luck