I did some more running on it and it seems that the external BEC has fixed the problem , what I was seeing was hesitation from the motor , the RX was staying on all the time and not going out . Changed settings too sensored only and now its smooth .
I have the same problem with the MMP and a Savox 1258 servo. (I tried 2s 3s and 4s - I always get this cutting out!) This has to be a software problem --- castle can you fix it ?