help me find an electrical gremlin, please

Do you by any chance have another distributor to try? I'm not suggesting you buy one, but IF you have an extra you could swap it out and see if that makes a difference. Just make a note of where the rotor is pointing before you pull it, drop the other one in the same way.

Run those two tests with your voltmeter and see what you come up with. Unfortunately when it cools down it may give you good readings which will further confuse you.

I drove a van for a previous employer some years ago that did exactly what yours is doing and my work had the shop look at it several times with no cure. The last time it did it it had to be towed back to the shop, I insisted the shop replace the pickup coil and problem solved.

IF it is the pickup coil you can buy just that piece and replace it.