Starts promptly from cold, even at low temps (20 F or so). After it reaches normal op temp and then turned off, it will start promptly but not hold idle (unless it has been off until it's cooled down). Holding the idle with the gas pedal, it eventually (5+ minutes) self-corrects. Any ideas? I have taken it to the best shop in town (which deals exclusively with high-end imported cars new and old), and they have failed to find a reason. Any hints?