#1: Are you using any extensions anywhere that have been used before?

#2: New switch or old?

#3: Is the antenna taped down anywhere?

#4: How close is the antenna to the rudder cables and/or servo extensions? Separate by as much as possible.

#5 How is the battery connection? New battery or o;d?

#6: Have you tried an electrical cleaner on the electrical contacts for all components?

#7: Are you using an old rx crystal in a new receiver?

#8: Has the transmitter ever been dropped? Check with a new crystal.

It's clearly not an engine/ignition issue since it also occurs with the engine not running.