Hi I have a mid-2011 27" iMac. Two days ago I was streaming video while working on my other computer. The mac switched itself off and then would not turn back on. I called up AppleCare, who advised me to take it in for a service. Meanwhile, I did some more googling and came across SMC resetting. I did this and hey presto, the mac turned back on. However, after about an hour of not particularly intensive stuff (copying files from internal HD to external HD, streaming music), it happened again. Once more, an SMC reset was necessary. This time, it switched off again during boot-up. I left it for half an hour, then booted in safe mode. I installed SMC fan control and Temperature Monitor. The mac stayed up (and stable, as far as I could tell) in regular mode (i.e. not safe mode) for around 24 hours. Today, I decided to do some stress-testing. I shut off SMC fan control and ran this: http://osxdaily.com/2012/10/02/stress-test-mac-cpu/ I watched the temperatures closely while this was happening. CPU activity was getting up to ~96% but none of the temperatures were particularly high. HDD was around 44deg C - the highest was the secondary heatsink which had just hit around 62deg C when the mac switched itself off again. I did an SMC reset (and a PRAM reset just for good measure) this time but it turned itself off while restarting again. All of this seems to point to overheating - but none of the temps seem particularly high. Does anyone have any ideas, or am I resigned to taking this in for a service? I'm still within my AC period, but I'd rather not be without my system for a week.