Moving cube

 I want a cube to move continuously by  z axis and when i press up arrow key i want the cube to translate by 0.3f in y axis and continue moving horizentaly but when i run the code the cube move horizentally as i want but when i press the up arrow key it  translates by more than 0.3f. Here's my code  

using UnityEngine; using System.Collections;

 public class Player : MonoBehaviour {
     
     Vector3 yy;
 
 
     // Use this for initialization
     void Start () {
         
         yy = transform.position;
     }
         
 
             
     // Update is called once per frame
     void Update () {
         
         transform.position += Vector3.forward * Time.deltaTime  ;
 
         if (Input.GetKey (KeyCode.UpArrow)) {
             yy = transform.position;
             yy.y =yy.y + 0.25f;
             transform.position=yy ;
             this.transform.Translate (new Vector3 (0, 0.3f, 0));
         }
 
          if (Input.GetKey (KeyCode.DownArrow)) {
             yy = transform.position;
             yy.y = yy.y - 0.25f;
             transform.position=yy ;
             this.transform.Translate (new Vector3 (0, -0.3f, 0));
         }
 
         Debug.Log (transform.position.y);
     }
 }