Any suggestions where to look first (and second and third)?
Check the 1/2" more or less molded air line below the throttle body for a soft spot that collapses under idle vacuum, choking off idle bypass air. The IAC opens up trying to maintain RPM, the engine looses vacuum due to almost stalling, the hose opens up, and the RPM goes up untill the IAC backs off again and the cycle repeats.
At least that's what caused it on mine. I found mine when the hose finally cracked making a vacuum leak (which then whistled). Found it when it started throwing codes.
Check for vacuum leaks and air leaks, and clean the throttle body and IAC
Sounds like a classic IAC problem. I've gone through 3 of them.
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