I've only noticed it when the car is warm. It does it in neutral and when in gear with the clutch depressed. Although I can feel / hear the idle changing, the needle on the tach doesn't seem to move at all. My car is used with just over 28,000 miles so I was wondering if it needed an injector cleaning or new spark plugs. But, given that cars that are essentially new are having the same issue, it seems to be a common characterisitic of this engine. - Jay