This happened to me a couple times in CB. Especially right after the last update and everyone and their dog was doing Campaign. It's usually followed by a Windows error and you get a message telling you Windows needs to close down the POL viewer.
I called POL and they couldn't tell me what the problem was especially since I had already checked the files for both the viewer and FFXI and there were no errors found. I havent had the problem again ever since I turn off options like weather effects and shadows to stop the overload while doing overpopulated events.