Also;

How is your throttle hooked up to the throttle servo?

Is the metal throttle horn ( the one on the carb ) hooked to a metal clevis?
In turn is the metal clevis hooked to a metal rod?

If so put in a plastic clevis.

The throttle rod is a great source of RF noise.

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I've had a couple of "bad" Hitec servos that would go nuts at certain throttle settings.

At first I thought I had a bad RF noise problem, coming from the engine.

I move the servo to another channel/surface, and the problem travelled with the servo.

I replaced the bad servos with two IDENTICAL Hitecs, and changed NOTHING else. The problem went away.

I've tested the bad servos on another airframe with identical results. I sent them back for exchange, and the replacements work fine.

So it is possible that you have a couple of servos that are problematic.