Motor seems steppy

[littlened]

For some reason the Speed Passion 6.5t I have in the Kyosho is very steppy, and it makes the car sound horrible at low speeds, like it's 'cogging'.

Here's a video

https://www.youtube.com/watch?v=9AbW...J7QudFRYO63oYj

I've got the spur and pinion setup to have a little bit of play, so they're not hard up against each other, but they're not too far apart that it can slip.

Does anyone have any idea what's going on? Have I setup the spur/pinion wrong?