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You're using a 411 PCM correct?
STIT is an idle short term trim similar to short term fuel trims, whereas it's not commanded in the tune but is rather a trimming based on the commanded base idle airflow and what is actually needed to maintain commanded idle.
Need to see if your ignition timing is dropping as a result of the idle rpm going down or if the idle timing drops first, then the rpm dips. Depending on what year OS is used in your car, there is a table that can be adjusted that controls how much timing is removed when the AC command kicks on, although that's different from just a second fan engagement setting.
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2000 ws6
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