I suspect that the throttle butterfly is not closing completely and the manifold vacuum isn't dropping fast enough. Suspect. I won't know until at least March when my garage temp gets warm enough to lift the hood and do some serious probing. The engine computer will compensate for the addditional air and adjust the fuel delivery, but that takes a few milliseconds.

If you remove the throttle body, there is a thin paper gasket that needs to be replaced. Just spray the TB with TB cleaner, using an old toothbrush for the stubborn buildup.

Because the IAC hose to the TB needs to be removed, clean the IAC with TB cleaner. Just spray it in, slosh it around, and pour out the dirty stuff. Once you hear a lot of metal-metal clacking while sloshing the TB cleaner around, it will be good to go.

Make sure that the hoses are in good shape, inspect the plastic vacuum tree, replace any nipple covers that are badly cracked. Note, the vacuum tree is old plastic an can get brittle with age. There is an O ring at the base, and if you pull out the tree, I suggest finding another O ring to make sure the seal is complete.
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My name is Klaus and I am a V♂lv♂holic