MouseLook initial rotation

I'm having an issue with a MouseLook script.

I've added a "if (Input.GetMouseButton(1))" in the Update function so that the "Look" portion of the script is only running when the right mouse button is being held. When I hold down on the mouse button, despite the initial camera orientation, the script jerks the camera rotation to (0,0,0).

Does anyone have any ideas on how I can keep my cameras rotation from being reset?

Here's the script:

 using UnityEngine;
 using System.Collections;
 
 /// MouseLook rotates the transform based on the mouse delta.
 /// Minimum and Maximum values can be used to constrain the possible rotation
 
 /// To make an FPS style character:
 /// - Create a capsule.
 /// - Add the MouseLook script to the capsule.
 ///   -> Set the mouse look to use LookX. (You want to only turn character but not tilt it)
 /// - Add FPSInputController script to the capsule
 ///   -> A CharacterMotor and a CharacterController component will be automatically added.
 
 /// - Create a camera. Make the camera a child of the capsule. Reset it's transform.
 /// - Add a MouseLook script to the camera.
 ///   -> Set the mouse look to use LookY. (You want the camera to tilt up and down like a head. The character already turns.)
 [AddComponentMenu("Camera-Control/Mouse Look")]
 public class MouseLook : MonoBehaviour {
 
     public enum RotationAxes { MouseXAndY = 0, MouseX = 1, MouseY = 2 }
     public RotationAxes axes = RotationAxes.MouseXAndY;
     public float sensitivityX = 15F;
     public float sensitivityY = 15F;
 
     public float minimumX = -360F;
     public float maximumX = 360F;
 
     public float minimumY = -60F;
     public float maximumY = 60F;
 
     float rotationY = 0F;
 
     void Update () 
     { if (Input.GetMouseButton(1)) 
     { if (axes == RotationAxes.MouseXAndY) { float rotationX = transform.localEulerAngles.y + Input.GetAxis("Mouse X") * sensitivityX;
                 
                 rotationY += Input.GetAxis("Mouse Y") * sensitivityY;
                 rotationY = Mathf.Clamp(rotationY, minimumY, maximumY);
                 
                 transform.localEulerAngles = new Vector3(-rotationY, rotationX, 0);
             }
             else if (axes == RotationAxes.MouseX)
             {
                 transform.Rotate(0, Input.GetAxis("Mouse X") * sensitivityX, 0);
             }
             else
             {
                 rotationY += Input.GetAxis("Mouse Y") * sensitivityY;
                 rotationY = Mathf.Clamp(rotationY, minimumY, maximumY);
                 
                 transform.localEulerAngles = new Vector3(-rotationY, transform.localEulerAngles.y, 0);
             }
         }
     }    
     void Start ()
     {
         // Make the rigid body not change rotation
         if (GetComponent<Rigidbody>())
             GetComponent<Rigidbody>().freezeRotation = true;
     }
 }