Upper 20% of screen goes dark - why?

With no evident stimulus or cause, the upper fifth or so of the screen will fade to black. It's not the television set or other accessory. Other video inputs work with no problem. 

We have Cox with Contour. Resetting the Cox box solves the issue, but it repeats at random.

Cox sends a reset signal when requested, and depowering-repowering works as well, but that's very annoying.

What could be causing this?