Thanks for the quick reply.
Yes, 100% consistent, unfortunately!
As far as I can tell, it seems to happen after a time, rather than be related to a specific throttle opening.
With the drone tethered I can increse the throttle past "hover" without any immediate glitch. It appears there's no obvious correalation between throttle and the problem. Then, after an inconsistent, but quite short period, the motor glitches.
There';s definitely NO mixing in my radio - set to Basic Aeroplane as per setup instructions. I did have to reverse the throttle channel.
I've calibrated the ESC's as per the online "all at once " instructions, and everything seems hunky-dory reponding exactly as it is supposed to.
I'm also running in "stabilize" (set all modes to this, to prevent finger trouble!).
I haven't yet configured additional switch channels (except 5 which is part of Calibration).
After my most recent session (yesterday) I did notice my Lipo was at a low state of charge - could this be triggering some kind of FAILSAFE operating on only 1 esc?
Seems unlikely, but...
Final question. I've configured the 3dr Quad kit as an "X" configuration, though its not strictly a symmetical X shape (the front & rear arms being in perfect alignment) . Should it be configured via Mission Planner as a "Y" .
I'll have another go this evening and double-check everything and use a fully-charged LiPo.