What's broken?

[insx]

Do you reckon this is the ESC, the motor or something else? The motor spins freely by hand.

https://youtu.be/zH4iKY6Dm1c

[guest325]

Looks a bit like it';s not getting the full three wires worth to me, looks like a phase has gone down either esc or connection.
Makes sure all of the joints are ok and check that the motor wiring is not damaged. If you';ve got one check it on a different  esc then try that esc on a different motor.

[Cheredanine]

Whilst I agree wi you a bit Darrell, in my experience when all three are not connected you get a more obvious stutter.
Don';t disagree with diagnosis though, although I would add make sure you motor screws are not just a little too long, had that before produce this sort of effect, back off the screws a turn or so, see if that cures the problem (it would also be apparent if the motor is getting hot)

[insx]

It was the ESC. On closer examination, the corner of the board had had a bash and a resistor was broken in half. I replaced the ESC and all is well.

[insx]

Wellish, anyway. The replacement ESC is not exactly like the old ones and doesn';t play nicely at calibration.