"Is your compass / gps unit mounted on a rod to keep it as far as possible away from the power circuits? It sounds very much like it's too near some sources of interference. Consider relocating it"

"As Philip says, looks like poor compass calibration...
Also look at performing a Compass MOT - and ensure compass is well away from sources of interference.."

"I did both Autotrim and Autotune James..

This time I'll connect my phone to the Nova whilst it performs the Autotune..  then at least I'll hear if it fails or is successful.. - maybe it failed last time...?
 
I'll get on to the mods this week and…"

"Thanks for the reply James...
I completely binned the stock GPS and compass and put a decent combo unit in the 'tower' in the Nova's case where the compass normally resides...
I get epic satellites and HDOP - so I was really confused as to cause of…"

"Well...in reply to myself.. I eventually found the solution.. so thought I'd place it here for closure's sake - and if anyone else has had problems with FRSKY X8R or X6R RX's

FRSKY eventually admitted there was an issue with the Euro version…"

"Thanks for taking the time to reply Graham!
Your mission video was bang on! - I've never had the balls to try it lol!
I have used the 'dronie' and 'follow me' commands from my phone via the Telemetry link though - they were fun!

So to be clear your…"

"No problem..

As I said above..   Also look at the Motor Test function under Initial Setup/Optional Hardware (Mission Planner) you can spin each motor independently. (From what I remember, they only turn clockwise though..)"

"Update!

I was brave enough to test the quad in the garden.
It armed in stabilise, and hovered very well. Suddenly there was what I can only describe as a pulse in one or more motors... and it landed.
I disconnected the gimbal power and tilt servo…"

"Looking at your video - it also seems like there's a lot of vibration...   have you checked your vibration levels..?
This can be the kiss of death for APM's.."

"This is def a mystery..!
It almost sounds like it's corrupt firmware...
I think that you should do as you suggested.. load a completely different firmware (maybe arduplane?)
Then re-flash back to quadcopter..  remember to select the correct…"

"Obviously you can check the direction of rotation of each motor...  that's the first thing to confirm, otherwise you'll experience the 'flip of death' as you try to take off
Then simply check the motor number against the diagrams and the APM output:…"

"Hi Graham,
Thanks for the clarification - I thought it would be ok...  After all it's only an APM..!
 
Could I ask?
Did you use the stock GPS unit and separate compass unit?
Does it fly ok in 'position hold' mode as opposed to 'loiter'?
Finally did…"

"Unfortunately on the CX-20 and Quanum Nova - It's a custom firmware... I'm not even sure the original firmware is available to allow you to revert back.
However, I see no reason why you cannot set it up as a normal Arducopter..  you may need to…"