Video Glitches Caught in the Act!
So ever since my iMac came back from the Apple Store, it hadn’t repeated the video glitches it was plagued with since March. Until now! I went to this very folder and clicked on the displayed video to play it in its Finder preview pane, and suddenly the glitches appeared!
They were pretty extensive, but they were also flickering very fast, and I was only able to get a screenshot of a sparse scattering. But now I have some clue as to what is setting them off.