frame grabber multithreading problem

Hi all

I m new in programming i have epiphan frame grabber which is connected to  my other computer to grab screen output i have downloaded SDK from epiphan in which there are few sample codes I have modified VC# code (initial code grab only one picture/screenshot but i have put the method in infinite loop to give it motion) problem is that when i call function(that grab frame) in infinite loop my application hangs and doesnt give any response. I created new thread and put that function in it now it works correctly but when when i comment following lines.

private void GrabFrame()

        {

            if (OpenGrabber()) {

                VideoMode vm = _grabber.DetectVideoMode();

                if (vm != null) {

                    if (vm.IsValid()) {

                        Frame frame = _grabber.GrabFrame();

                        if (frame != null) {

                          //trigger=_grabber.SetMaxFps(1000.0);

                          //  _grabber.Start();

                            Bitmap bitmap = frame.GetBitmap();

                            if (bitmap != null) {

                                //TextOutputAdd("Frame captured (" + vm + ")");

                            //"Frame captured (" + vm + ")"

                                // TextOutput.Text = "Frame captured (" + vm + ")" + "\r\n" + TextOutput.Text;

                                PictureBox.Image = bitmap;

                            } else {

                               // TextOutputAdd("Capture format error");");//(display message in text box)

                                //SetGrabber(null);

                            }

                        } else {

                            //TextOutputAdd("Capture failed");");//(display message in text box)

                            //SetGrabber(null);

                        }

                    } else {

                        //TextOutputAdd("No signal detected");");//(display message in text box)

                    }

                } else {

                    //TextOutputAdd("Failed to detect video mode");//(display message in text box)

                   // SetGrabber(null);

                }

            }

        }

But when I uncomment following lines following error appears

An unhandled exception of type 'System.InvalidOperationException' occurred in System.Windows.Forms.dll

Additional information: Cross-thread operation not valid: Control 'TextOutput' accessed from a thread other than the thread it was created on.

Code for button that run thread

private void th_instance() {

            for (; ; )

            { GrabFrame();

}}

private void CaptureFrame_Click(object sender, EventArgs e)

        {

            //=====================================================

            th1 = new Thread(new ThreadStart(th_instance));

              th1.Start();

        }

 sample source code is attached

i will highly appreciate if somebody could guide me to solve issue and provide me best solution

regards