A video would make things clearer. I doubt the Rx8 has weak BEC issues but to rule them out, can you install a " glitch buster" and tell us if it works out? If not, it's probably the radio or the ESC but I would bet on the chances of the radio being off.
Can you try it out with another radio or Rx by any chance?
It's nice to see someone ask questions in a polite manner.
