Sintered rotor unstable at low speeds? Normal?

[Fiddybux]

I've got an X11 5.5 sintered motor here and a 2007 Sphere.

I've had this problem where the car seems to stop every so often for no apparent reason.

I've just got the motor out of the car but all connected and at low RPM the motor speed is unstable. I'm doing this by setting the drive ratio to say 5% on the transmitter.

You can clearly hear it juddering and speeding up and down. Sometimes it even momentarily stops - like for a fraction of a second. Is this normal?

I don't have this issue with a bonded rotor - the one that originally came with the X11 motor. I've tested tonight and compared the two and the bonded one is fine.

ESC settings are set to, ADPC number 3, 0 initial brake and 0 auto brake. The ADPC makes light difference when set lower, but the motor still exhibits this strange behaviour.

Any ideas? Thanks.