สอบถามเรื่องการ Random ตัวเลขเมื่อเรากดปุ่มหยุดแล้วให้ Random ช้าลงเรื่อยๆ แล้วก็หยุดไป

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsRandom
{
    public partial class Form1 : Form
    {
        private int _duration;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            buttonStart.Enabled = true;
            buttonStop.Enabled = false;

            textBox1.ReadOnly = true;
            textBox2.ReadOnly = true;
            textBox3.ReadOnly = true;
            textBox4.ReadOnly = true;

            textBox1.BackColor = System.Drawing.Color.White;
            textBox2.BackColor = System.Drawing.Color.White;
            textBox3.BackColor = System.Drawing.Color.White;
            textBox4.BackColor = System.Drawing.Color.White;
        }

        private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
        {
            while(_duration > 0)
            {
                Random random = new Random();

                int[] r = new int[] { random.Next(0, 9), random.Next(0, 9), random.Next(0, 9), random.Next(0, 9) };

                backgroundWorker1.ReportProgress(0, r);

                System.Threading.Thread.Sleep((1000 - _duration == 0) ? 100 : 1000 - _duration);

                if (_duration < 1000)
                {
                    _duration -= 200;
                }
            }
        }

        private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            int[] r = (int[])e.UserState;

            textBox1.Text = r[0].ToString();
            textBox2.Text = r[1].ToString();
            textBox3.Text = r[2].ToString();
            textBox4.Text = r[3].ToString();
        }

        private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            buttonStart.Enabled = true;
            buttonStop.Enabled = false;

            textBox1.BackColor = System.Drawing.Color.White;
            textBox2.BackColor = System.Drawing.Color.White;
            textBox3.BackColor = System.Drawing.Color.White;
            textBox4.BackColor = System.Drawing.Color.White;
        }

        private void buttonStart_Click(object sender, EventArgs e)
        {
            _duration = 1000;

            buttonStart.Enabled = false;
            buttonStop.Enabled = true;

            textBox1.BackColor = System.Drawing.Color.LightGray;
            textBox2.BackColor = System.Drawing.Color.LightGray;
            textBox3.BackColor = System.Drawing.Color.LightGray;
            textBox4.BackColor = System.Drawing.Color.LightGray;

            backgroundWorker1.RunWorkerAsync();
        }

        private void buttonStop_Click(object sender, EventArgs e)
        {
            _duration = 800;

            buttonStart.Enabled = false;
            buttonStop.Enabled = false;
        }
    }
}