Orca spark problem

[RichH]

Hi
I've bought a used orca spark esc motor combo. It runs smoothly when I disconnect the sensor wire but when I connect the wire again it runs erratically, on a constant throttle the revs drop then jump up again and when you give it full throttle it goes mental reving really fast and sticking there for a few seconds even when the throttle is released. I've rebound the TX rx and reset the esc but its still the same? Any help would be appreciated

Cheers
Rich