screensaver issue with dual monitors

My screensaver has recently started acting up. It's sort of a minor blip, but it worked before and now it won't. Here's the issue: I have dual monitors (Intel imac, running Leopard, with a Viewsonic 20" lcd running in extended desktop mode). I use a "smart" album from iphoto as a screensaver (iphoto 08, v7.1.5). I've been using this particular smart album for several months as my screensaver and it's worked fine. How it used to work: when the screensaver activates (either by time or by hot corner) both screens would draw photos from this album and randomly scroll, zoom, and fade them across each monitor, independently but simultaneously. It's worked like this flawlessly for years, with several different iphoto albums. Now, I just noticed that the second monitor does not find the pictures. A message on screen "looking for pictures" appears on both monitors, as normal, but instead of both monitors going to screensaver, only the imac starts, and the message "looking for pictures" stays on the other. What's weird is that if I run the screensaver in test mode, it works like it used to (both screens work independently and simultaneously). And if I switch to one of Apple's supplied screensavers, both screens work as well. But any iphoto album no longer works properly. I don't understand what has changed. I just recently ran an upgrade (Software Upgrade, a week or so ago) but it seems like it worked normally since then, and I don't believe the upgrade was a major one or had anything to do with iphoto. I just noticed this problem today. I've tried restarting the machine. Same issue. Any ideas what could be causing this minor, yet annoying, glitch.