could be the fuel screws not right, could be the needles not right, tough to tell when I cant see or hear the sled run.
if it does it more on take offs and down low, I would go to the fuel screws. (likely not out enough)
if it does it when your running a steady 30-40mph and give it more throttle and it kinda babbles along, its the needle settings.
this is precisely why it pays to learn how to tune and set up your sleds yourself, because when a kid or someone does it at the dealer, you dont know the specs or where its set at, and then the guessing begins.