I doubt it's your IAC because once the vehicle is warmed up, it has very little effect. It could be a sticking EGR valve. Try disconnecting and plugging the vacuum to the EGR when it's cold, and see if the problem goes away. If it does, replace the EGR. If it doesn't, try removing the ECM chip, and see if that makes a difference. My money is on the EGR.
Take care,
~Chris
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