are you using a metal clevis at the engine control arm that is also metal? That could be your problem.
try a plastic one.

Also balance your prop, check every inch of the plane for metal to metal contact.

How old is the battery, I've seen old batteries cuase interferance just before the solder came loose in them and failed

I've seen metal pins used to attach the reciver antenna cause interferance.

I've seen tailwheel set up that have cause interferance when slighly loose.

I've also seen long servo connections, thoe as long as the reiciver anntena tpo be specific cause interferance. This is usually only a problem with large gliders though and can be remidied by putting a 3" extension on or a servo filter in line witht he long servo lead.