I've been having a problem with my aging Lombard Powerbook. A couple weeks ago, I booted it up only to get a circle with a slash through it (like a no smoking sign). I reinstalled the OS and all seemed well for a day or two, when I got the circle with a slash through it again. I tried reinstalled the OS again, and sure enough, a few days later it was toast again. Any clue what could be causing this?