We have a 27" iMac (2011). Just recently it has started powering down on it's own. When you try to restart the iMac with the power switch nothing happens. You have to unplug the power cord, wait several hours and when you plug it back in you can power it on again. This has happened 3 times in the past week. With the exception of this, we have had zero problems with the iMac and are still really happy with it. Any ideas on what could be the problem and if this would be repairable versus buying a new computer?