By sheer luck, I figured out what was happening. With the 64GB card, the last dozen videos were shot at 30 frames per second. I set the shutter speed at 1/60 and the videos were coming out looking good. When I put in the new 256GB card, I shot 7 videos using the 1/60 shutter speed and all were horrible.
This afternoon, I was fiddling around with the settings and unknowingly changed the shutter speed to "Auto" rather than 1/60. Bingo! I can't explain the reason but all 6 videos turned out beautifully. Why the Sandisk Extreme gladly accepted 1/60 shutter speed and produced nice videos while the Sandisk Extreme Pro revolted against it remains a mystery.
In the end, there are now three video settings I have that will all do well.
Thanks for your help with this!!