what kind of servo are you guys using for throttle?
i ask because i use to have this issue with different engines, and traced it to the servo.
i use Futaba radios and found the standard cheap servos S3004 or S148's would have not so good centering. what happens is the servo never finds the same spot when throttled down. i now use S9001's or other precision servos and get the same exact idle drop when throttling down.