I _think_ it was every other frame, and I am fairly sure that it was a software/driver issue. It had never happened before, it has not happened since, and it started immediately when starting the game and the symptoms got progressively worse and triggering the mild symptoms happened every time I started the game.

What I convinced myself of after a few minutes being sure I wasn't hallucinating was that the graphics driver was pushing out malformed data in some way or the other which was triggering bugs in the monitor hardware/firmware, which is easy to believe are plentiful. It would be an interesting project to try to track down and replicate the bug.