G'day Lawrence,good to hear from you,I've only just got back from an unplanned trip.Okay if you've checked for the usual suspects and found nothing,then have a look at the plug lead for any damage or the plug itself for any cracks in it,also your Rx battery leads to the switch and rx for any damage caused through rubbing.If they are ok,do an inspection on your crystal as it might be cracked or it may have come loose.
I have my butterfly valve setup so that it's fully open at full throttleI will chase up my throttle/Pitch setting's for you as they are not to hand at the moment.
Are your collective pitch servo and throttle servo matched for the same speed as if the CP servo is faster the the throt servo you will have head snatching and it will give the impression that the throttle/pitch setting's are outit's the same with the throt servo being faster than the pitch servo you end up with the engine bogging down when throttling up due to the pitch servo not keeping up with the throt servo.

cheers,
glenn.