It looks like you have flaps on it? and in the picture they appear to be down some what and the elevator is huge I wonder if the diehedrial combined with some down flap combined with a large elevator movement pitches the nose up and it goes into a "air brake" mode and slows down fast enough to self right, by pulling the air away from the ailerons?
Hi, thanks for the input. I did go 'around the houses' on the flaps/ailerons combinations; flat wing, under-camber wing, washout with the ailerons... without any progress, but very importantly what I did get from the replies was that (as I suspected) this was quite an unusual problem.
One of the responses on transmitter mixes (which I checked, and they were all fine) got me thinking about the receiver and servos. A quick visit to YouTube threw up a pile of videos where either a dodgy or even 'different' servo caused motion and fluttering in other servos. I may be wrong (I haven't had a chance to test fly it yet) but it could be a dodgy servo firing back interference to the other servos, or, I strongly suspect, the receiver is faulty - cross/firing input from one servo to other servos under certain load conditions. So as not to waste time, and risk 'unnecessary' crashing
,I've replaced both the receiver and the servo.
Fingers crossed, and I'll post my results.