Player movment: height and straight besides normal xz axis?

Hello,

I 've met the situations:

1 when i move my player in xz-plane, after awhile the y-position starts changing?

2 when i try to move my player in xz-plane and i cross the plane (not along x- or z- axis), i can not move it in straight line forward?

I 'm moving via a script:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine.UI;
 using UnityEngine;
 
      
 public class bootcontroller : MonoBehaviour {
    
 public Rigidbody rb;
 public transform ;
 public float speed = 17;
 //good test script
 
 Vector3 rotationRight = new Vector3(0, 30, 0);
 Vector3 rotationLeft = new Vector3(0, -30, 0);
 Vector3 forward = new Vector3(1, 0,0);
 Vector3 backward = new Vector3(-1, 0, 0);
 void FixedUpdate()
 {
     if (Input.GetKey("up"))
     {
         transform.Translate(forward/2 * speed * Time.deltaTime);
     }
     if (Input.GetKey("down"))
     {
         transform.Translate(backward/2 * speed * Time.deltaTime);
     }
     if (Input.GetKey("right"))
     {
         Quaternion deltaRotationRight = Quaternion.Euler(rotationRight * Time.deltaTime);
         rb.MoveRotation(rb.rotation * deltaRotationRight);
     }
     if (Input.GetKey("left"))
     {
         Quaternion deltaRotationLeft = Quaternion.Euler(rotationLeft * Time.deltaTime);
         rb.MoveRotation(rb.rotation * deltaRotationLeft);
     }
 }}

Thx, Rob