新线程启动新form，在主线程中，修改新form的控件值，引起主界面卡死，如何解决～

主界面代码：
//主界面程序
        private void ShowProgress()
        {

            PB = new ProgressBar();
            PB.ShowDialog();
        }
        private void button1_Click(object sender, EventArgs e)
        {
                int val = 1;
                ProgressThread = new Thread(new ThreadStart(ShowProgress));
                ProgressThread.IsBackground = true;
                ProgressThread.Start();
                do
                {
                    val += 1;
                    Thread.Sleep(100);
                }
                while (PB.SetInvokePGB(val));
            }
        }
//进度条界面程序
    public delegate void InvokeControl(int str);
    public partial class ProgressBar : Form
    {
        public ProgressBar()
        {
            InitializeComponent();
        }
        public bool SetInvokePGB(int val)
        {
            if (progressBar1.InvokeRequired)
            {
                if (val < progressBar1.Maximum)
                {
                    BeginInvoke(new InvokeControl(SetPGB), val);
                    return true;
                }
                else
                {
                    BeginInvoke(new InvokeControl(Close), val);
                    return false;
                }
            }
            else
                return false;
        }
        public void SetPGB(int val)
        {
                progressBar1.Value = val;
        }
        public void Close(int val)
        {
            this.Close();
        }
    }
运行过程中，为什么主界面会卡死，请问如何处理？ --------------------编程问答-------------------- private void ShowProgress()
        {

            PB.ShowDialog();

        }

        private void button1_Click(object sender, EventArgs e)
        {

            ProgressThread = new Thread(new ThreadStart(ShowProgress));
            ProgressThread.IsBackground = true;
            PB = new ProgressBar();
            ProgressThread.Start();
            Thread TD = new Thread(new ThreadStart(SS));
            TD.Start();
        }

        public void SS()
        {
            int val = 1;
            do
            {
                val += 1;
                Thread.Sleep(100);
            }
            while (PB.SetInvokePGB(val));
        } --------------------编程问答-------------------- 应该是线程间上下文切换引起的

补充：.NET技术 ,  C#