[JohnH]

it shouldn't, actually. The ECU should compensate for idle going below and above certain rpm. AC turning on, etc will make the idle lower, but the PCM should raise it. Usually with timing adjustments, or throttle adjustments. Back in the day, they used idle air control devices, but I don't think that ours has one of those being that it has drive by wire electronic throttle.

Id say to clean the throttle body really well, and check for air leaks/rips in the intake pipe.

I'd also look at my fuel trims at idle. If its overly lean or overly rich, that would be a clue