It is not the ESC that is faulty, it is the BEC unable to handle the 4 cells and is stealing power from the ESC causing the sputter or wierd pulsating motor. In fact mine has the stock ESC and new BEC from castle creation. I simply cut the red wire from the BEC to receiver and wired the castle to the battery lead to the receiver batt servo plug. An new ESC with a BEC that can handle 4cell batt should do. On mine, the BEC can now handle up to 6cell batt.