Camera Script - RtS style, rotation issues

Here is my script:

 using UnityEngine;
 using System.Collections;
 
     
     public class CameraRTS : MonoBehaviour
     {
     
         public int ScrollSpeed = 100;
      
         public int ZoomSpeed = 25;
         public int ZoomMin = 5;
         public int ZoomMax = 100;
      
         public int PanSpeed = 50;
         public int PanAngleMin = 25;
         public int PanAngleMax = 80;
         
         public string forward = "z";
         public string backward = "s";
         public string left = "a";
         public string right = "d";
         
         //variabiles for rotating the camera
         public enum RotationAxes { MouseXAndY = 0, MouseX = 1, MouseY = 2 }
         public RotationAxes axes = RotationAxes.MouseXAndY;
         public float sensitivityX = 250;
         public float sensitivityY = 250;
     
         public float minimumX = -360;
         public float maximumX = 360;
     
         public float minimumY = -270;
         public float maximumY = 90;
          
     
         
         // Update is called once per frame
         void LateUpdate()
         {
             // Init camera translation for this frame.
             var translation = Time.deltaTime*ScrollSpeed;
         
             // Zoom in or out
             var zoomDelta = Input.GetAxis("Mouse ScrollWheel")*ZoomSpeed*Time.deltaTime;
             if (zoomDelta!=0)
             {
                 //In order to Zoom, there is some limits
                 if(camera.fieldOfView - ZoomSpeed * zoomDelta > ZoomMin && camera.fieldOfView - ZoomSpeed * zoomDelta < ZoomMax ){
                     camera.fieldOfView -= ZoomSpeed * zoomDelta;
                 }
             }
             
             // Move camera with arrow keys
             if (Input.GetKey(left)){
             var movement = new Vector3 (-translation, 0, 0);
             transform.Translate(movement, Space.World);
             }
             if (Input.GetKey(backward)){
             var movement = new Vector3 (0, 0, -translation);
             transform.Translate(movement, Space.World);
             }
             if (Input.GetKey(right)){
             var movement = new Vector3 (translation, 0, 0);
             transform.Translate(movement, Space.World);
             }
             if (Input.GetKey(forward)){
             var movement = new Vector3 (0, 0, translation);
             transform.Translate(movement, Space.World);
             }
             
             //Rotate the camera while pressing "Backspace" key MUST BE FIXED
             if(Input.GetKey(KeyCode.Backspace)){
                 
                 float rotationY = 0F;
     
                 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);
                 }
         
     
                 
                 
             }//end "Backspace" section
             
             
         }//LateUpdate
         
     }//Class

The camera moves correctly, zoom ok. But when I have to rotate it (while pressing Backspace), it is impossible to view anything: looks like an earthquake is happening!

Some suggestions?