I recently asked for help for screwy behaviour of the APM board, but after some digging, it turns out that it was the Spektrum DX6i Transmitter which was causing all the issues.
I was getting erratic flights and crashes but when I moved to a DX7s, all my issues went away. I then noticed a complete failure of the DX6i Elevator Control Stick
Looking online, I found that this is a know issue. Spektrum are recalling and repairing the issue for those with the bad pots, but you need to take the controller to a Spektrum dealer for return.
There is a batch number on their website, but mine failed even though it wasnt listed.
Worth checking tho.