Here's a bit of an update.
It seems to be some sort of electrical issue.
Here's a video with the motor out, and I've set the transmitter to apply a little throttle, you can hear how it sound so steppy.
http://www.youtube.com/watch?v=B1_dCnNXrDE
At one point it even increases in speed.
Now I've had this problem since I got the car, and in that time I've had a different ESC and motor. I'm pretty certain I've also changed the transmitter, but I can't remember.
Could this be because the transmitter is crap? It does the same thing even with another motor in.
I've turned off reverse on the ESC as well just incase it was that.
Anyone any ideas?