Memory Leaks in windows media player ActiveX control?

There is a memory leak bug in my winform application.

In order to find out the cause, I maked a simple test application to build the bug. The application has only one Form, the Form has a Button and a windows media player ActiveX control named "axWindowsMediaPlayer1". Parts of the codes is shown below:

        private void Form1_Load(object sender, EventArgs e)
        {
            this.axWindowsMediaPlayer1.uiMode = "none";
            this.axWindowsMediaPlayer1.settings.mute = true;
            this.axWindowsMediaPlayer1.settings.autoStart = false;
            this.axWindowsMediaPlayer1.enableContextMenu = false;
            this.axWindowsMediaPlayer1.windowlessVideo = true;
            this.axWindowsMediaPlayer1.fullScreen = false;
            this.axWindowsMediaPlayer1.settings.setMode("loop", true);
        }
        private bool m_bLoopFlag = false;
        private void btnLoop_Click(object sender, EventArgs e)
        {
            m_bLoopFlag = true;
            Thread thd = new Thread(new ThreadStart(this.LoopPlayThread));
            thd.IsBackground = true;
            thd.Start();
        }

        private void LoopPlayThread()
        {
            string strUrl = "";
            for (int i = 0; i < 50000; i++)
            {
                strUrl = @"http://192.168.1.101/video/a.mp4";
                this.Invoke(new MethodInvoker(delegate
                {
                    PlayUrl(strUrl, true);
                }));
                if (!m_bLoopFlag) return;
                Thread.Sleep(3000);
                strUrl = @"http://192.168.1.101/video/b.mp4";
                this.Invoke(new MethodInvoker(delegate
                {
                    PlayUrl(strUrl, false);
                }));
                if (!m_bLoopFlag) return;
            }
        }

        private void PlayUrl(string url, bool startPlay)
        {
            IWMPMedia media = axWindowsMediaPlayer1.newMedia(url);
            if (axWindowsMediaPlayer1.currentMedia != null && !axWindowsMediaPlayer1.currentMedia.get_isIdentical(media))
            {
                axWindowsMediaPlayer1.Ctlcontrols.stop();
                axWindowsMediaPlayer1.currentPlaylist.clear();
            }
            axWindowsMediaPlayer1.currentMedia = media;
            if (startPlay)
            {
                axWindowsMediaPlayer1.Ctlcontrols.play();
            }
        }

I have found some key points to build the problem.

1. The media played by wmp must be a net mp4 media, in my test application, it's placed in a IIS virtual directory.
2. The size of the mp4 file is large enough(>300MB ?).

My machine's system info: 

OS 32Bit Windows 7 Ultimate with SP1
Processor Intel(R) Core(TM) i5 CPU         750  @ 2.67GHz, 2668 Mhz, 4 Core(s), 4 Logical Processor(s)
Total Physical Memory 2.96 GB
Display  NVIDIA GeForce 205    Driver Version 9.18.13.697

May anyone tell me what's the problem of my code, or any suggestions?
Best regards