UPDATE
I discovered what was causing the problem with sporadic blacked out fence roll downs. It was apparently a downloaded desktop wallpaper image that was causing this. Changing the wallpaper and I inadvertently made the problem go away. Brought the wallpaper up again and down come the blacked out fence rollups. Yes, that wallpaper is no longer on my computer. Being computer semi literate, I haven't a clue how the wallpaper and Fences interacted to cause this, but am thankful it's gone.
Basj, thank you for responding.
Best,
KK