Twice in the past few days when I return to my computer there is a message in three languages over a gray sq. with a power icon: "You need to restart your computer. Hold down the Power Button for several seconds or press the Restart Button." after I restart another message saying unexpected quit and do i want a report. When I click the report icon no report appears. Nothing else seems odd while I am working but did have to take my Mac Mini to the Apple Store a month or so ago after I kept getting "?" at start up. I am using 10.4.7. My question: is this a fluke, something more serious, and anything I should do?