There are no codes being stored. I wish it were storing codes, as that would help tons. He swapped out the air flow meter (at now charge) and it made no difference. He's monitored the engine while it's doing it and just just shows a variance in the idle speed and (I think) the fuel mixture. (The norm is something like 50%, but varies going as high as 60% at times.) He has tried finding the intake manifold leak by spraying something non-flammable like brake cleaner.

I've gotten to know the mechanic (he's the best there) and the dealer well... and I think they know what they're doing and charging a fair price. Any ideas would be much appreciated!