I have problems with walking in unity, please help!

I can move my character and I can move the camera with the cursor, but every time, the character starts spinning around after around 10 seconds after I started the game. Here is the script:

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof (Rigidbody))]
 [RequireComponent (typeof (CapsuleCollider))]
 
 public class RigidbodyFPSWalker : MonoBehaviour {
     
     public float speed = 10.0f;
     public float gravity = 10.0f;
     public float maxVelocityChange = 10.0f;
     public bool canJump = true;
     public float jumpHeight = 2.0f;
     private bool grounded = false;
 
     public GameObject fpsCam;
     public GameObject PeaMachineGun;
 
     public int health = 100;
     public GameObject me;
     
     
     void Awake () {
         GetComponent<Rigidbody>().freezeRotation = true;
         GetComponent<Rigidbody>().useGravity = false;
     }
     
     void FixedUpdate () {
 
         if (grounded) {
             // Calculate how fast we should be moving
             Vector3 targetVelocity = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
             targetVelocity = transform.TransformDirection(targetVelocity);
             targetVelocity *= speed;
             
             // Apply a force that attempts to reach our target velocity
             Vector3 velocity = GetComponent<Rigidbody>().velocity;
             Vector3 velocityChange = (targetVelocity - velocity);
             velocityChange.x = Mathf.Clamp(velocityChange.x, -maxVelocityChange, maxVelocityChange);
             velocityChange.z = Mathf.Clamp(velocityChange.z, -maxVelocityChange, maxVelocityChange);
             velocityChange.y = 0;
             GetComponent<Rigidbody>().AddForce(velocityChange, ForceMode.VelocityChange);
             
             // Jump
             if (canJump && Input.GetButton("Jump")) {
                 GetComponent<Rigidbody>().velocity = new Vector3(velocity.x, CalculateJumpVerticalSpeed(), velocity.z);
             }
 
 
         }
 
         if(health <= 0)
         {
             GetComponent<PhotonView>().RPC("DIE", PhotonTargets.AllBuffered, null);            
         }
         
         // We apply gravity manually for more tuning control
         GetComponent<Rigidbody>().AddForce(new Vector3 (0, -gravity * GetComponent<Rigidbody>().mass, 0));
         
         grounded = false;
     }
     
     void OnCollisionStay () {
         grounded = true;    
     }
     
     float CalculateJumpVerticalSpeed () {
         // From the jump height and gravity we deduce the upwards speed 
         // for the character to reach at the apex.
         return Mathf.Sqrt(2 * jumpHeight * gravity);
     }
 
 
 
     void OnGUI()
     {
         GUI.Box (new Rect(10, 10, 100, 30), "HP | " + health);
     }
 
     [PunRPC]
     public void applyDamage(int dmg)
     {
         health = health - dmg;
         Debug.Log ("hit" + health);
     }
 
     [PunRPC]
     public void DIE()
     {
         if(GetComponent<PhotonView>().isMine)
         {
             Destroy (me);
             GameObject.Find ("_Room").GetComponent<RoomManeger> ().spawnPlayer ();
         }
     }
 
 
 
 
 
 
 }