I had written several posts a few weeks back on this subject. I had one computer where I could do external monitor previews, another where I couldn't. I tried everything, even reformatting and reloading the errant machine from scratch.
I believe I solved the problem. I deleted the video driver and rebooted, making sure that only the native Winbloze VGA driver was being used. Then I deleted the IEEE 1394 Bus Host Controllers driver and rebooted. Then I loaded the latest version of my video card driver and rebooted, then loaded the OHCI Compliant IEEE 1394 driver instead of the Texas Instruments IEEE 1394 driver and rebooted.
Now I have external monitor preview without large pixel blocks flashing in and out. I'm not sure which of these solved the problem but I'm guessing it was replacing the IEEE 1394 driver. Worked for me, hopefully it will work for others.
I believe I solved the problem. I deleted the video driver and rebooted, making sure that only the native Winbloze VGA driver was being used. Then I deleted the IEEE 1394 Bus Host Controllers driver and rebooted. Then I loaded the latest version of my video card driver and rebooted, then loaded the OHCI Compliant IEEE 1394 driver instead of the Texas Instruments IEEE 1394 driver and rebooted.
Now I have external monitor preview without large pixel blocks flashing in and out. I'm not sure which of these solved the problem but I'm guessing it was replacing the IEEE 1394 driver. Worked for me, hopefully it will work for others.