I fly a couple of instances of the Neu motor with the Castle ESC's and am by no means an expert on pattern electric stuff -- starting 4th electric season. Having said that have you looked at the motor/gear shaft when it is turning to see if there is any run out? The same for the spinner backplate relative to the nose ring; and the nose/cone of the spinner? Might look at the tracking of the prop tips, also.

It's not prop flutter is it? Frustrating for sure. Post the findings please for the rest of us! and good luck with it!