Stuttering for the first second or so

[littlened]

Kyosho ZX-5
Novak 7.5 brushless motor (bought second hand but not used much)
LRP SPX ESC, brand spanking new
Acoms 2.4ghz receiver and tranmitter
5100 Orion NIHM batteries

I'm not sure exactly what's wrong, all I can explain is that on forward and reverse, the car seems stuttery for the first second or so, then it takes off.

With the motor out of the car and not connected to anything, it seems to run smoothly enough.

I've tried a brand spanking new 9.5t brushless motor (LRP), and it does the same.

Anyone any ideas?