Hi everyone,

Does anyone have any suggestions for the cause of this? When I start from cold, the engine idles smoothly. After about 5-10 min - just enough time for the temp gauge to get to normal and for me to get out of the neighborhood to the first light - the idle becomes unsteady: every 4-5 seconds, the rpms drop precipitously, only to recover quickly. After another 10-20 mins of driving, this goes away completely, and the engine idles smooth as silk for as long as I keep driving.

I recently had the throttle body cleaned, thinking it would address this, but no joy. I have checked for vacuum leaks, too. No CEL codes appear. I can monitor MAF, O2 sensor, vacuum, fuel flow, LTFT, STFT etc with Torque pro app. Nothing seems really awry. Seems like it could be some sort of temp sensor, since it only happens in a certain temperature range.

Anny suggestions would be greatly appreciated!

I would try cleaning out the idle air control, the round thing in front of the throttle and under the throttle cover. Just take it off, spray a bunch of throttle body cleaner inside, shake it vigorously, and pour out the black liquid a couple of times.

Make sure that the IAC hoses aren't cracked and leaking air. Also check that the electrical plug isn't corroded. The IAC should humm when you turn the key to position II, before engine start.
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Keeping it running is better than buying new

Thank you, I will have a look at the IAC. But wouldn't that cause uneven idle at any engine temperature? Can a dirty IAC or cracked hose cause a problem only at a specific temperature? I am still suspicious of some temp sensor... but the one that reports to the dash indicator seems ok, at least it starts out at the bottom and rises slowly to horizontal, where it stays when the engine is warm. Is there another one that could be bad?

1996 850 5-speed, 222K miles, still strong! (but the paint is kinda bad, never garaged in 23 years).

Because of the dash temperature gauge being somewhat vague, it is possible that the engine coolant temp sensor is slow to react. Has it ever been replaced?

There is a way to check it out with a multimeter. Unplug the ECT and then probe it with a multimeter. The colder the engine, the higher resistance number you will get. At normal operating temp, it should be around 250. At 60F it should be around 1200. Hard to get a 32F reading these days.
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Keeping it running is better than buying new