JD 4520 error code 14

[HHFarm]

My 4520 has been serviced at least 10 times under warranty for error code 14 problems. At least 3 times it was in the shop for a week. The tractor will not go forward but at times it will go in reverse if you hold the peddle down long enough (HST Drive). I not sure if I got a "lemon" or this is a common issue with the 4000 series HST John Deere Tractors? Anyone seeing similiar problems? My tractor was towed, once again, back to the shop 4 days ago.