How can you prevent slowing down if you look up/down while using FPS controller?

I'm using the standard asset FPSInputController/charactermodel scripts. I notice when I look up or down the forward/backward movement speed crawls to a halt. Somewhat embarrassing to say but I couldn't figure out how to keep the motion fluid. Anyone have some simple suggestions?

I think the answer lies in adjusting the motor Quaterion to align to a 2d plane before applying the directionVector.

Here's the code:

 function Update () {
     // Get the input vector from kayboard or analog stick
     var directionVector = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
     
     print("direction vector is: " + directionVector);
     
     if (directionVector != Vector3.zero) {
         // Get the length of the directon vector and then normalize it
         // Dividing by the length is cheaper than normalizing when we already have the length anyway
         var directionLength = directionVector.magnitude;
         directionVector = directionVector / directionLength;
         
         // Make sure the length is no bigger than 1
         directionLength = Mathf.Min(1, directionLength);
         
         // Make the input vector more sensitive towards the extremes and less sensitive in the middle
         // This makes it easier to control slow speeds when using analog sticks
         directionLength = directionLength * directionLength;
         
         // Multiply the normalized direction vector by the modified length
         directionVector = directionVector * directionLength;
     }
     
     // Apply the direction to the CharacterMotor
     motor.inputMoveDirection = transform.rotation * directionVector;
     
     motor.inputJump = Input.GetButton("Jump");
 }