I had an issue once with a 1.20FZ that drove me and everyone else I knew nuts. The engine would behave quite well on the ground most times when pulling from a high throttle to idle and would usually quit in the air doing the same thing unless I pulled back very slowly. The culprit was a piece of grass that had been sucked in and was stuck to the throttle butterfly in the intake pipe on the side you cannot see and was partially obstructing the airbleed for the idle needle. It's a long shot but might be worth pulling it out for a little look and see if something is blocking that hole. I dumped out a gallon of fuel because I thought it was a bad batch that was causing my problem.