I had an problem like that with my 1.20FZ last year. Drove me nuts. I changed the check valve, diaphram, and even dumped out my fuel and opened a new jug. At the suggestion of friend I took the carb apart and found a piece of grass on the cylinder pipe side of the butterfly an some gunk in the idle needle. Runs fine now, but like you I couldn't duplicate the problem on the ground, only in the air after throttle up after a downline.