I once did a test shot of a notepad document (with just A's on word wrap) on my monitor... zoomed in so it was the only thing visible (no monitor edges to go off of) and it hunted like no tomorrow (would go OoF on wide, past 'in focus', then OoF on zoom, then repeat). I was further than min focusing distance away (so that wasn't it). I attribute it to many factors, most of which it being a light source, distance, ambient lighting, the refresh rate of the monitor, and the camera settings. It was something I normally don't have an issue with, but was able to create (and duplicate) a perfect storm of settings where I could trigger it on demand.