Must be something in your system, because there's nothing jerky about it here or on any other computer I've used. Putting a first-person camera script in FixedUpdate is not the right thing to do...that will make it only update when the physics system updates, instead of every frame. Try turning off any mouse hacks or utilities you might be using.

maybe its the computer thats running a bit slow?

Searching the forums I found the following script and it solved my FPS camera movement (I’ve made some adaptations to the original script to fit my needs. Sorry, I don't know who the author is).

    using UnityEngine;

using System.Collections;

using System.Collections.Generic;



public class SmoothMouseLook : MonoBehaviour

{

    /*

    This script is used to average the mouse input over x 

    amount of frames in order to create a smooth mouselook.

    */



    //Mouse look sensitivity

    public float sensitivityX = 2f;

    public float sensitivityY = 2f;



    //Default mouse sensitivity

    public float defaultSensX = 2f;

    public float defaultSensY = 2f;



    //Minimum angle you can look up

    public float minimumY = -60f;

    public float maximumY = 60f;



    //Number of frames to be averaged, used for smoothing mouselook

    public int frameCounterX = 35;

    public int frameCounterY = 35;



    //Mouse rotation input

    private float rotationX = 0f;

    private float rotationY = 0f;



    //Used to calculate the rotation of this object

    private Quaternion xQuaternion;

    private Quaternion yQuaternion;

    private Quaternion originalRotation;



    //Array of rotations to be averaged

    private List<float> rotArrayX = new List<float> ();

    private List<float> rotArrayY = new List<float> ();



    void Start ()

    {

       //Lock/Hide cursor

       Screen.lockCursor = true;



       if (rigidbody)

         rigidbody.freezeRotation = true;



       originalRotation = transform.localRotation;

    }



    void Update ()

    {

       if (Screen.lockCursor) {

         //Mouse/Camera Movement Smoothing:   

         //Average rotationX for smooth mouselook

         float rotAverageX = 0f;

         rotationX += Input.GetAxis ("Mouse X") * sensitivityX;



         //Add the current rotation to the array, at the last position

         rotArrayX.Add (rotationX);



         //Reached max number of steps?  Remove the oldest rotation from the array

         if (rotArrayX.Count >= frameCounterX) {

          rotArrayX.RemoveAt (0);

         }



         //Add all of these rotations together

         for (int i_counterX = 0; i_counterX < rotArrayX.Count; i_counterX++) {

          //Loop through the array

          rotAverageX += rotArrayX[i_counterX];

         }



         //Now divide by the number of rotations by the number of elements to get the average

         rotAverageX /= rotArrayX.Count;



         //Average rotationY, same process as above

         float rotAverageY = 0;

         rotationY += Input.GetAxis ("Mouse Y") * sensitivityY;

         rotationY = ClampAngle (rotationY, minimumY, maximumY);

         rotArrayY.Add (rotationY);



         if (rotArrayY.Count >= frameCounterY) {

          rotArrayY.RemoveAt (0);

         }



         for (int i_counterY = 0; i_counterY < rotArrayY.Count; i_counterY++) {

          rotAverageY += rotArrayY[i_counterY];

         }



         rotAverageY /= rotArrayY.Count;



         //Apply and rotate this object

         xQuaternion = Quaternion.AngleAxis (rotAverageX, Vector3.up);

         yQuaternion = Quaternion.AngleAxis (rotAverageY, Vector3.left);

         transform.localRotation = originalRotation * xQuaternion * yQuaternion;

       }

    }



    private float ClampAngle (float angle, float min, float max)

    {

       if (angle < -360f)

         angle += 360f;

       if (angle > 360f)

         angle -= 360f;



       return Mathf.Clamp (angle, min, max);

    }
    }

Steps to use it: Add a First Person Controller to the scene and remove the Mouse Look script all together 1) Attach Smooth Mouse Look to the Character Controller with this parameters:

Sensitivity X: 4 Sensitivity Y: 0 Default Sens X: 4 Default Sens Y: 0 Minimum Y: 0 Maximum Y: 0 Frame Counter X: 20 Frame Counter Y: 20

2) Attach another Smooth Mouse Look to the Main Camera with this parameters:

Sensitivity X: 0 Sensitivity Y: 5 Default Sens X: 0 Default Sens Y: 5 Minimum Y: -60 Maximum Y: 60 Frame Counter X: 20 Frame Counter Y: 20

That’s it!

I have the exact same problem as the person wrote above. I once had Unity on an old machine and it was a bit jumpy then. I now have a Core i3 computer with Intel HD graphics and it is very jerky. I have gone through all the graphics settings but it hasn't changed anything.