Have you added external JS or other code to the page within the last few months? I don't know why, but sometimes proximity to certain JS files executing nearby within the layout can cause a freakout in the AdSense display size. This happened to me with a social media widget.
I sometimes see something like that. On my site, it happens right after a new page with an unrelated iframe is posted online. An AdSense ad will appear in the unrelated iframe for some reason. Usually goes away quickly, meaning a few refreshes of the page. .