Three times in the last week or so my iMac's screen has just turned white (once with almost a ribbed pattern with thin grey vertical stripes evenly interspersed) and become completely unresponsive, with the only option being a hard reset. It's happened under a variety of conditions: while surfing in Safari, while working in Aperture, and once without doing anything (I was on the phone sitting by my computer). Each time I've restarted it everything seems to work fine for a couple days, but then it happens again. I've never seen anything like this and I figured after the third time it's time to ask for help. Does anyone have any ideas?