OK, here's the news: I just went out and tried it again and it
stumbled to a stall in about a minute.

Then I disconnected the O2 sensor. It ran strongly for five minutes or
so, longer than at any other time today. Then while it was running, I
plugged it back in and it was dead in 30 seconds.

I will try to repeat this again in a few minutes to confirm what I
think just happened.

Jim