I had the same thing, i have the same speedo as you along with an LRP x12 motor. I fitted a Novak motor instead and bingo, problem solved. I sent the X12 motor back to JE spares who sent it back to the suppliers. I got the motor back last week with a letter saying the motor is fine, no fault found???
seems strange how i have no cogging with any of my novak motors yet as soon as the X12 is wired into the car using the same speedo and you put it under load it coggs like goodun'.
I changed everything, settings on the speedo, the speedo itself, (changed to a hobbywing, lrp tc spec and a speed passion), sensor leads, motor timing even fitted the electrics into a different car and with the X12 motor it coggs.
GGRRRRRR