@carusoam Finally a probable solution. It wasn't a bad field diode; it was a completely absent field diode! Diagnosed by DMax via email, by the way.
Reviewing the logs, the previous owner had replaced the stock Continental alternator with a Plane Power unit. A few months later, the ACU was sent out for "repair". I have no idea the reason for the repair, and the shop where it was done did not have access to the work order. My guess is that the diode was removed with the swap and maybe the ACU was sent out for the FB popping, but that is only a guess.
The diode was added.
The very next landing the FB popped and the idiot light on the annunciator panel showed that it was due to overvoltage. I had never checked regarding over/under voltage before that time and thought that `o..sh` it didn't work. But the next six landings have all been OK with no further popping of the field breaker.
So, hopefully, the good behavior will continue.