photon78s Perhaps something else is the cause that has intermittent behavior.
In my experience, some panels temporarily disable pixel inversion under certain conditions. For instance, for some panels, I can only see the pixel inversion when the image only takes up a small part of the screen. And once I display a large image, it isn't enough to make the image smaller - I have to make the image smaller, then maximize a different window (like I am somehow resetting the screen) and then bring up the smaller image and then I can see the pixel inversion. And it isn't just my eyes playing tricks on me because the photodetector is detecting the same thing.
That's a pretty specific example and it might not apply to your situation, but it does indicate that pixel inversion can be intermittent.