2D multiple direction movement with arrows problem

Hello,

I am creating a 2D horizontal shmup and I can't find what's the problem with movement.

1. Movement without firing works fine.

2. Movement with firing (spacebar) with keys WASD works fine.

3. Movement with firing (spacebar) with arrow keys when moving x axis positive (right) + y axis positive (up) works fine. Same with both axes negative.

4. Movement with firing (spacebar) with arrow keys when axes are opposite (I.E. x negative, y positive) doesn't work. The character moves in single direction.

Here is the code (problem is probably within Update() function):

 using UnityEngine;
 using System.Collections;
 
 public class PlaneParameters : MonoBehaviour {
     public float fireRate = 0;
     public float Speed = 2;
     public float movementSpeed = 2;
     public Transform bulletPrefab;
     private float timeToFire = 0;
     private Transform firePoint;
 
     void Awake (){
         firePoint = transform.FindChild("FirePoint");
         if (firePoint == null){
             Debug.LogError ("Can't find Plane child FirePoint");
         }
     }
 
     void Start (){
     }
 
     // Update is called once per frame
     void Update () {
 
         // movement with camera
         transform.Translate(Vector3.right * Speed * Time.deltaTime);
 
         //player input movement
         if (Input.GetButton ("Horizontal")){
         transform.Translate(new Vector3 (Input.GetAxis("Horizontal") * movementSpeed * Time.deltaTime,0f,0f));;
         }
 
         if (Input.GetButton ("Vertical")){
         transform.Translate(new Vector3 (0f,Input.GetAxis("Vertical") * movementSpeed * Time.deltaTime,0f));
         }
 
         //restrictions within screen
         var pos = Camera.main.WorldToViewportPoint(transform.position);
         pos.x = Mathf.Clamp01(pos.x);
         pos.y = Mathf.Clamp01(pos.y);
         transform.position = Camera.main.ViewportToWorldPoint(pos);
 
         //firing
 
         if (Input.GetKey (KeyCode.Space) && Time.time > timeToFire){
             timeToFire = Time.time + 1/fireRate;
             Instantiate(bulletPrefab, firePoint.position, firePoint.rotation);
         }
     }
 
     void OnCollisionEnter2D(Collision2D col) {
         if (col.gameObject.tag == "Enemy")
             {
             Application.LoadLevel("restartScreen");
             }
         }
 }
 

 

Any ideas what's wrong?

Thanks in advance.