Sky Toro and weird motor problem!?!

[Jraw36]

Hi All i have a sky Toro TS120 speed controller connected to a Thunder tiger 6.5t motor.

Now when I connect the sensor lead to the motor (and i've tried 3 different leads) and give it throttle the motor feels and sounds gritty, also if i give it full throttle it starts medium speed and after a second or two it winds up to max, instead of instant full throttle.

Now when I disconnect the sensor lead the motor runs smooth and on full throttle gets instantly up to speed.

Whats going on here??

Any help much appreciated.

Thanks all