Yeah, if the throttle servo is moving, it's not an engine problem. (unless something is loose on the engine)
Look for metal-to-metal contact. Remove it. Make sure to use plastic clevis for throttle,
make sure no metal is touching the head, etc... Alternately, if you find nothing else, I'd
begin to suspect you may have a bad servo, bad solder joint at the antenna, broken antenna
where it enters the receiver case, or a bad crystal.