First, make sure your 780 is up to date. There's a new software release on the Quantum Data website just a week ago that has a lot of new features that might be useful for you, such as a new installer tools menu.
You can also try playing some of those clips into the 780 and see if you can see the video on the 780 using its Video Display feature, to see if maybe it's outputting the video, but in a format that the display doesn't support.
You can also try playing some of those clips into the 780 and see if you can see the video on the 780 using its Video Display feature, to see if maybe it's outputting the video, but in a format that the display doesn't support.