I'm not sure if this will help in your particular case, but I was experiencing someting similar on my older SHARP T.V It turned out the fix in my case was to turn DYNAMIC CONTRAST (in Picture Settings) to OFF.
It appears that the implementation of Dynamic Contrast in the current ROKU firmware, at least in my set, was not very good and the result was that the entire picture would brighten or darken annoyingly. Dynamic Contrast is supposed to enhance the viewing experience, not destroy it.
