Issue around the 16 minute mark. It usually occurs after a throttle change (cruise speed adjustment) or when initially entering cruise mode. Moving the throttle around can eventually get it to a point where it stops oscillating again. Or changing flight modes.
The only part of the plane I haven't changed now are the ESCs and the airframe itself. The ESCs are fast response BLHeli ESCs that I had from another project - would there be any chance the faster throttle response could cause an issue?
I've cycled through everything else and redone the hinging, control rods and horns.
It's just odd that no one else seems to have the same issue except for possibly that one parrot disco.
I wish I could do more to help. I'd be happy to trawl through code but I wouldn't have a clue what to look for