Zen Cart 1.3.8a

I've had zen cart running perfectly for over a year now, but suddenly my product images have begun disappearing. Nothing has changed, no new addons, upgrades, nothing. But random product images just stop appearing on the page, usually only one at a time, out of over a hundred or so total images. This has been happening at the rate of about once per day. The images are still on the server, but I have to re-load them from zen cart admin/product page.

Is this a known issue? Anyone have any ideas about why this would be happening?