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).