Shut if off when the idle is high and try to slide a piece of paper or thin feeler gauge in between the idle speed screw and the throttle stop to make sure it is closing all the way. I had a similar problem that ended up being the throttle bell crank was binding with the choke arm. If I eased off the throttle it would bind. If i snapped the throttle and let it shut quickly it would have enough momentum to work past the bind and fully close