LRP X12 motor running problems

[Team No Idea]

Hey Chaps, Im having a bit of a nightmare getting ready for maritime's Off Road Wars this coming weekend.

I ordered a new 8.5 LRP X12 motor and fitted it last night to my Nosram pearl istc. I have run this car with the nosram every week for a few months with a Novak Ballistic 13.5 with no problems. However now ive fitted the LRP the car judders like buggery (i think its known as cogging) form initial take off. So i tried replacing the sensor wire, i altered the motor timing, reset the speedo, and adjusted the throttle end points on the transmitter.........Its still messing around. So i change the speedo to my spare Hobbywing 120a, still hte same issue. So i change the motor to my 10.5 LRP and my Nosram pure 5.5 motors, still the cogging issue.
So finally i install my 13.5 novak and hey presto it works!
Any ideas chaps?