During my second year of owning a late 2012 iMac 27" I found it started turning itself off. Unfortunately I didn't have any AppleCare so just put up with it. Since the new year though I've had it in with a local computer shop twice to see if they could solve the issue. I'd googled the issue and found several people with the same problem but multiple different answers. Common one was overheating so I tried the MacFan app but even set to maximum that made no difference. First time in the shop, they found the insides and fan caked with dust so cleaned it all out and tested it, seemed fine. Trouble is the fault is very intermittent - it can go off after one hour or work fine nearly non-stop for days. Second time, they thought it was definitely the power supply so got a new one from Apple and fitted it. Again, seemed fine but a few days later it went again. They want to test it again but I'm starting to wonder if it will ever be fixed. Every time it happens, I have to unplug the plug, leave it for 5 minutes then plug in and turn on. If I don't do this, it will not re-start. it doesn't seem to matter if I have multiple things running or nothing. It can be after one hour or eight. Do any of you have experience with this issue or any solutions or answers?