Hi! I was reading this forum for years and now... i registered because a problem (ouch!) ... I wonder if someone can help.... My iMac shuts down randomly since i installed Yosemite. It always happen when i'm doing something, sometimes opening Safari, sometimes launching an App... the system doesn't shut down if you don't touch it. I made my investigations.... - I re-install Mavericks and Snow Leopard and the problem remains. So is not related with Yosemite at all. - Apple Hardware Test (the long one) says there's no problem with any piece of hardware (the iMac didn't shuts down while performing the test). - There's nothing plugged into the mac, so the problem is not related with peripherals. - There's no pattern into the system log. The last messages recorded before the system shuts down are always different (no errors). - The temperature registered in all sensors are fine, all the fans are working properly. - I have an UPS unit so the current should be nice and protected. So... Can all the shuts down be only fault of the Power Supply Unit? I can switch on the mac after the shut down without any problem. There's a way to test the PSU before i pay for a new one (expensive part!)? Any idea of something different causing all the trouble? Thanks!!