Y axis is changing randomly

I am making a 2d platformer type game and on the player controller script (it is like the one from the live training)
whenever I walk the y axis kind of bounces, it goes up by about 0.3 to 0.4 every time the an arrow key is pressed:

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class PlayerController : MonoBehaviour {
5.      
6.         [HideInInspector] public bool facingRight = true;
7.         [HideInInspector] public bool jump = false;
8.         public float moveForce = 200f;
9.         public float maxSpeed = 1f;
10.         public float jumpForce = 500f;
11.         public Transform groundCheck;
12.  
13.  
14.  
15.         private float lockPos = 0;
16.         private bool grounded = false;
17.         private Animator anim;
18.         private Rigidbody2D rb2d;
19.      
20.      
21.         // Use this for initialization
22.         void Awake ()
23.         {
24.             anim = GetComponent<Animator>();
25.             rb2d = GetComponent<Rigidbody2D>();
26.         }
27.      
28.         // Update is called once per frame
29.         void Update ()
30.         {
31.             //locks position values
32.             transform.rotation = Quaternion.Euler(transform.rotation.eulerAngles.z, lockPos, lockPos);
33.  
34.             grounded = Physics2D.Linecast(transform.position, groundCheck.position, 1 << LayerMask.NameToLayer("ground"));
35.          
36.             if (Input.GetButtonDown("Jump") && grounded)
37.             {
38.                 jump = true;
39.             }
40.         }
41.      
42.         void FixedUpdate()
43.         {
44.             float h = Input.GetAxis("Horizontal");
45.          
46.             anim.SetFloat("Speed", Mathf.Abs(h));
47.          
48.             if (h * rb2d.velocity.x < maxSpeed)
49.                 rb2d.AddForce(Vector2.right * h * moveForce);
50.          
51.             if (Mathf.Abs (rb2d.velocity.x) > maxSpeed)
52.                 rb2d.velocity = new Vector2(Mathf.Sign (rb2d.velocity.x) * maxSpeed, rb2d.velocity.y);
53.          
54.             if (h > 0 && !facingRight)
55.                 Flip ();
56.             else if (h < 0 && facingRight)
57.                 Flip ();
58.          
59.             if (jump)
60.             {
61.                 anim.SetTrigger("Jump");
62.                 rb2d.AddForce(new Vector2(0f, jumpForce));
63.                 jump = false;
64.             }
65.         }
66.      
67.      
68.         void Flip()
69.         {
70.             facingRight = !facingRight;
71.             Vector3 theScale = transform.localScale;
72.             theScale.x *= -1;
73.             transform.localScale = theScale;
74.         }
75.     }

also the z rotation would go to 90 whenever i walked so I put in
transform.rotation = Quaternion.Euler(transform.rotation.eulerAngles.z, lockPos, lockPos);
and that has stopped that problem but I would like to know what caused it in the first place,

Thanks

 

BUMP