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How much fluctuation are you talking about here? 50rpm, 400rpm, almost stalling?
Think about the things the affect running condition at idle, none of the things you have done have anything to do with it. Fuel trims should be around +/- 5% or so. 10% is very high. AFR at cruise doesn't tell us anything about idle. It's either vacuum leak, metering, or a load is being placed on the engine like AC or the alternator as mentioned above. It's a random thing on some cars, as long as it is not too severe it is a "feature." There are tons of threads on it.
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"Experience is the hardest kind of teacher. It gives you the test first and the lesson afterward." -Oscar Wilde.
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