Sounds exactly like RFI to me. It's probably the cap, as already suggested.

If that don't fix it........... Is the elevator servo new? What about the servo extension going from the RX to the elevator servo?

I once had a Y harness go bad on a glow plane. It acted just like RFI. Was causing the ailerons to jump around randomly when the engine was on.

Just an idea, but I'll bet the hose clamp on the cap fixes it.