I need help!

I have a HW Xerun 60a/13.5 combo and at super low throttle,my motor is nice and smooth. At about 1/4th throttle, the RPM's jump around like the motor is revving up and down, and past 1/4 throttle, the motor is nice and smooth again. Why is it doing this? I have loaded three different firmwares and it still does the same thing. I used the old 110stock non-supercharge firmware, 422stock,and 518stock and the same thing. Do I have a bad sensor wire? Please help! Thank you...