Thank you very much for your response and for your invaluable contributions to this forum!
You're quite right, the AAV was the first thing I addressed, it used to not close fully thus causing the idle speed to continue changing in the operating temperature range. The warm idle used to have to be set to ~1300 rpm to be able to idle at all after a hot highway run!
After servicing, the AAV now is open only when the engine is cold and is fully closed by 80°C, checked by putting a thumb over the intake to the valve -lots of vacuum when cold, no suction at all after a little warm-up. Now the idle in neutral is stable at 850-900 rpm after a short warmup, and remains so even after the engine gets hot. Even when I encounter the heat-soaked stalling problem with engaging drive, the idle in neutral is still 850-900 rpm.
I'll try the series resistor on the temp sensor. I take it you mean the coolant sensor, and not the air sensor, yes? Is it reasonable to try simply unplugging this sensor to see if the stall disappears, or does that make it way too rich?
If this resistor corrects the behavior, is this a good long-term fix, or do I need to replace the sensor?