Perhaps a complete false trail but did they do an intake leak check?

I had a similar issue with a 944 a while back. Visually everything was fine but there was a crack in an elbow feeding into the intake after the mass flow sensor. Engine movement would open and close this randomly. It was most noticeble at idle with symptoms much like you describe. Otherwise the engine management system pretty well masked it until it got bad enough for it to throw a sensor fault.

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Les