If you want to use the PIDController class for this, be sure to use the FeedFoward term. It is documented partway down this page:
http://wpilib.screenstepslive.com/s/...rs-pid-control
but basically, you need to give the motor a base value that is close the desired motor speed. The P, I, and D adjustments will be added to the FeedForward term in the calculations.
Brad