Thanks both of you for educating me. I'm still not sure what to do although
when I get a moment I am going to try to test the TPS Voltage -- if my
limited technical skill and analog tester allow. In the meantime, let me
pose a question that I've had for months:

At some point during warm-up, the ECU changes its outgoing instructions from
"warm-up mode" to "operate normally." This is the exact moment my problem
always starts. What is going on in the sensor communication to make that
change happen? Which component can do its job just fine and idle at exactly
1K during warm-up, but drops the ball and falters below 750 when the
instructions change? In my non-tech interpretation, that's the culprit I'm
after. It seems so clear cut because when I'm having the problem it always
plays out exactly the same way.

Any ideas? Thanks again for taking time on my problem.