Simon,

My contribution to your problem. I'd change one thing at a time so you know if one was the problem.

- Check all electrical connections for a good connection and no shorts.
- Change motor and see if problem goes away.
- Check to make sure the pushrods and surfaces are not binding.
- Ensure servos are working smoothly and are in good repair.
- Prop balanced?
- Is shaft straight and gearbox working smoothly (Great planes suggests to put a piece of paper between the spur and the gear when assembling their gearbox. I start the screws for the motor, 'roll' a piece of paper in, and then tighten up the screws, then roll the paper out).
- Test the battery both after it is charged and discharged to see if the charger or the ESC may be the problem.