We have this unit that all tech's agreed that it sounded like drive gears were toast. We disassembled the drive case, A/C traction motor and found no damage whatsoever. Replaced bearings and races. Disassembled brake and everything is in specs. After reassembly the same condition persists. Drive chatters like it's grinding gears. No fault codes and it seems that something is cutting out. Can't get it out of turtle mode, has anyone ever experienced this condition? If you can help thanks.