Make sure all of the motor wires have a solid connection. Maybe you have one hanging on by just a tiny bit of solder. I had a similar thing happen on my first brushless set up where it used bullet plugs from the motor to the speedo. After spreading the prongs on the plugs a bit to make a tighter connection, the cogging was gone. Might be something else entirely, but no one else has suggested anything, so it's something to check anyway.