the usual culprit tends to be a weak BEC in the ESC combined with a "large" current drain from the servo
the usual supects are savox servos and spektrum RXs ( not exclusively however ) reading similiar posts those two names crop up regularly
a power cap pluggesd into a spare RX port can help eliminate this
no guarantees on success
i have mainly spektrum rx and some savox servos and have occasionally suffered this ( running 1s lipo, spekky rx and savox servo is a deffo no-no and requires a lipo booster
Fid