I have a 27" iMac and it has recently started shutting down all by itself. It used to be that I could use it and then when it went into sleep mode it would shut down. But I tried to power it up today and it just shuts right back down. It's less than a year old and I have an appointment at the apple store later today, but I was curious if anyone has had a similar issue. It also starts up really slow. I bought it to replace an older PowerPC G5 and I'm thinking I should have stuck with that.