Originally Posted by ColSonders
It looks like the ESC is knackered then.
I'd suggest that it's an issue with the electronics on the quad itself if the issue follows the ESC channel for a specific motor.
I don't see how it could be anything else given the tests done.
Swapping channels on the RX eliminates the RX as the same behaviour still occurs.
Multiple TX including none computerised radio eliminates the radio.
At this stage the issue has been narrowed down to the Quad itself.
Switching motors on ESC has shown the motors to be fine.
This leaves either the ESC board has an issue or as someone else stated earlier it could be the Flight Controller board having an issue with the balancing and compensating not calculating correctly.
I'd be asking the manufacturer for a replacement ESC board if it's in warranty.
Again confirmed that the aircraft itself without any problems, including the ESC and FC.
Check the channel of the remote control correctly, including aircraft connection, please read the instructions carefully.
Manufacturers confirm please with the remote control, remote control factory model, whether it is a fixed-wing mode? Is mixing?