RE: Interference: replaced everything and still there??
To be completely clear the shielded metal plug cap grounds the sparkplug internally and it is grounded to the ignition module only? No other ground connections exist?
Try the Futaba Tx with a PPM Rx, you never know…
It’s unlikely the engine is the problem, especially if it’s an electronic ignition system. The ignition is always a consideration, but today’s modern stuff is good and you mentioned trying a second unit.
Sounds like you have eliminated and or replaced everything? Has any component always been present in the model electrically?
Always run the model up with the wings in place, the wings dampen the engine considerably.