Unity 3D Screen.lockCursor Problems

Hi guys, i'm having problems with locking my cursor in the middle of the screen, even when i have it set true, it moves about, please help heres my script:

 var mainMenuSceneName : String;
 var pauseMenuFont : Font;
 private var pauseEnabled = false;            
 
 function Start(){
     pauseEnabled = false;
     Time.timeScale = 1;
     AudioListener.volume = 1;
     Screen.showCursor = false;
     Screen.lockCursor = true;
 
 }
 
 function Update(){
 
     //check if pause button (escape key) is pressed
     if(Input.GetKeyDown("escape")){
     
         //check if game is already paused        
         if(pauseEnabled == true){
             //unpause the game
             pauseEnabled = false;
             Time.timeScale = 1;
             AudioListener.volume = 1;
             Screen.showCursor = false;    
             GetComponent(MouseLook).enabled = true;    
             Screen.lockCursor = false;
             
         }
         
         //else if game isn't paused, then pause it
         else if(pauseEnabled == false){
             pauseEnabled = true;
             AudioListener.volume = 0;
             Time.timeScale = 0;
             Screen.showCursor = true;
             GetComponent(MouseLook).enabled = false;
             Screen.lockCursor = false;
         }
     }
 }
 
 private var showGraphicsDropDown = false;
 
 function OnGUI(){
 
 GUI.skin.box.font = pauseMenuFont;
 GUI.skin.button.font = pauseMenuFont;
 
     if(pauseEnabled == true){
         
         //Make a background box
         GUI.Box(Rect(Screen.width /2 - 100,Screen.height /2 - 100,250,200), "Pause Menu");
         
         //Make Main Menu button
         if(GUI.Button(Rect(Screen.width /2 - 100,Screen.height /2 - 50,250,50), "Main Menu")){
             Application.LoadLevel(mainMenuSceneName);
         }
         
         //Make Change Graphics Quality button
             if(GUI.Button(Rect(Screen.width /2 - 100,Screen.height /2 ,250,50), "Change Graphics Quality")){
             
             if(showGraphicsDropDown == false){
                 showGraphicsDropDown = true;
             }
             else{
                 showGraphicsDropDown = false;
             }
         }
         
         //Create the Graphics settings buttons, these won't show automatically, they will be called when
         //the user clicks on the "Change Graphics Quality" Button, and then dissapear when they click
         //on it again....
         if(showGraphicsDropDown == true){
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 ,250,50), "Fastest")){
                 QualitySettings.currentLevel = QualityLevel.Fastest;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 50,250,50), "Fast")){
                 QualitySettings.currentLevel = QualityLevel.Fast;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 100,250,50), "Simple")){
                 QualitySettings.currentLevel = QualityLevel.Simple;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 150,250,50), "Good")){
                 QualitySettings.currentLevel = QualityLevel.Good;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 200,250,50), "Beautiful")){
                 QualitySettings.currentLevel = QualityLevel.Beautiful;
             }
             if(GUI.Button(Rect(Screen.width /2 + 150,Screen.height /2 + 250,250,50), "Fantastic")){
                 QualitySettings.currentLevel = QualityLevel.Fantastic;
             }
             
             if(Input.GetKeyDown("escape")){
                 showGraphicsDropDown = false;
             }
         }
         
         //Make quit game button
         if (GUI.Button (Rect (Screen.width /2 - 100,Screen.height /2 + 50,250,50), "Quit Game")){
             Application.Quit();
         }
     }
 }