Continues to have an intermittent misfire, and stalling, and difficulty starting or no-start during warmup phase.
From cold-start condition:
Started the truck, as it warmed up, I kept the idle up over 2500, and it tried to stall several times, and once or twice did, but I was able to restart and get the rpms up quick enough.
Once it fully warmed up (as measured by coolant temp), it stabilized and ran well enough (still has an intermittent 'misfire'). Drove it around the block, appeared to run well enough (like it has the past year, with an occaional misfire).
Restarted it several times. Each time it would:
Initially run smoothly at just-started advanced idle (initially about 1500 rpm, dropping to about 1000-1200 (didn't write it down)). After about 30 seconds, it would drop to about 800 rpm, and simultaneously begin slight intermittent misfire, and under the hood would be two sounds, a 'humming' and a slight 'rattle' sound. After another 30 or so seconds, the humming and rattle would quit, and the intermittent misfire would continue.
I recall reading a few posts about a 'humming', so I'm going to turn my attention there for the moment.
I also strongly suspect the O2 sensor, which I've also read (if I remember correctly) can act up during warmup, but be fine at cold start and full operating temp.
More as I learn...