Unity3D 坂道を上がるとYの値が増えてしまい重力が逆向きに上がってしまうコードの修正法

坂道を上がりキーを離すとその分？上に上がってしまうバグの原因と修正方が知りたいです(提示画像参照)。下にさがる重力の仕様があるのでAddForceは消したくありません。

提示コード(プレイヤースクリプト)

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Player : MonoBehaviour
{
    const float walk_speed_max =  20.0f;

    Rigidbody rb;
    Vector3 move;
    float input_h;
    float input_v;
    Vector3 move_x;
    Vector3 move_z;

    public GameObject gGround;
    ground spt_g;
    bool isJump;
    bool isDush;
    bool isRay;
    bool isGround = false;
    float jump_force = 200.0f;
    float move_speed = walk_speed_max;
    Animator ani;

    private Transform StepHit;
    private Transform Hit;
    private RaycastHit ray;

    const float ray_range = 0.3f;//レイを飛ばす距離

    //　昇れる段差
    private float stepOffset = 0.3f;

    //　昇れる角度
    private float slopeLimit = 65f;

    //　昇れる段差の位置から飛ばすレイの距離
    private float slopeDistance = 0.1f;
    private Vector3 v;
    // GameObject go = GameObject.Find("GameObject");
    private Transform prev;

    void move_step()
    {                                                           //y軸は逆にしてるプラスが下 
        Debug.DrawLine(transform.position + new Vector3(0f, stepOffset, 0f), 
                transform.position + new Vector3(0f, stepOffset, 0f) + transform.forward * slopeDistance
                , Color.green);
        //     Debug.Log(transform.forward * slopeDistance);

        float s = Mathf.Sqrt((move.x * move.x) + (move.z * move.z));

        if (s > 0)
        {
            if (Physics.Linecast(StepHit.position, StepHit.position + StepHit.forward * ray_range, out ray))
            {

                if (Vector3.Angle(transform.up, ray.normal) <= slopeLimit)                
                {

                    v = new Vector3(0f, (Quaternion.FromToRotation(Vector3.up, ray.normal) * transform.forward * s).y, 0f)
                        + transform.forward * s;
                    move.y = v.y;
                    isRay = true;
                    Debug.Log("レイ");
                }
                else
                {
                    isRay = false;
                    //         move.y = 0;
                    //move += transform.forward * move_speed;
                }

            }
        }
    }

    // Start is called before the first frame update
    void Start()
    {
        rb = GetComponent<Rigidbody>();
        // spt_g = gGround.GetComponent<ground>();
        ani = GetComponent<Animator>();
        isJump = false;
        isDush = false;
        StepHit = transform.Find("StepRay");

    }

    // Update is called once per frame
    void Update()
    {
        //input_h = Input.GetAxis("Horizontal");
        //input_v = Input.GetAxis("Vertical");

        if(Input.GetKey(KeyCode.LeftArrow))
        {
            input_h = -1;
        }else if (Input.GetKey(KeyCode.RightArrow))
        {
            input_h = 1;
        }
        else
        {
            input_h = 0;
        }

        if (Input.GetKey(KeyCode.UpArrow))
        {
            input_v = 1;
        }
        else if (Input.GetKey(KeyCode.DownArrow))
        {
            input_v = -1;
        }
        else
        {
            input_v = 0;
        }

        move_z = Vector3.Scale(Camera.main.transform.forward, new Vector3(1, 0, 1)).normalized * input_v;
        move_x = Camera.main.transform.right * input_h;
        move = move_x + move_z;
        move.x *= move_speed;
        move.z *= move_speed;

        Debug.Log(isRay);
        if (move != Vector3.zero)
        {
            transform.rotation = Quaternion.LookRotation(move.normalized);
        }

        /*走る*/
        if(Input.GetKey(KeyCode.RightControl))
        {
            isDush = true;
            move_speed = 15.0f;
        }
        else
        {
            isDush = false;
            move_speed = walk_speed_max;
        }

        if(Input.GetKey(KeyCode.Space))
        {
            if(isGround == true && isJump == false)
            {          
                isJump = true;
            }
        }

        move_step();
        Animation_Mng();
    }

    /*アニメーション管理*/
    void Animation_Mng()
    {
        float speed = Mathf.Sqrt((move.x * move.x) + (move.z * move.z));
        ani.SetFloat("Move_Speed",speed);
        ani.SetBool("isDush",isDush);
        //ani.speed = 2;
    }

    private void FixedUpdate()
    {
        //  rb.velocity = new Vector3(move.x, rb.velocity.y, move.z);

        if(isJump == true)
        {
            //rb.AddForce(rb.velocity.x,jump_force,rb.velocity.z);
            isJump = false;      
        }

        /*坂道判定のrayが衝突処理*/
        if (isRay == false)
        {
            rb.velocity = new Vector3(move.x, rb.velocity.y, move.z);
            rb.AddForce(new Vector3(0, -10.0f, 0));
        }
        else
        {
            //rb.AddForce(new Vector3(0, -10.0f, 0));
            if(move.y == 0)
            {
                rb.velocity = new Vector3(move.x, -100, move.z);
            }
            rb.velocity = new Vector3(move.x, move.y, move.z);
        }

    }

    private void OnCollisionEnter(Collision c)
    {
        //isGround = true;
        //   Debug.Log("プレイヤーcollision");
    }

    private void OnCollisionExit(Collision c)
    {
        //isGround = false;
    }

}