I have recently faced a critical issue with my iMac (since June 2016). The problem showed up after installing OSX El Capitan. It shutdown randomly and suddenly. It's as if the power cord has been disconnected. And it doesn't restart when I press the power on button. I have to switch off the power socket, wait for a few minutes, then it can be turned back on.
After 3 visits to Apple Store to fix this, they ran same test to see if it's a hardware- or softwareproblem (negative), the ram-module is replaced, they have done a clean install and it's still the same problem.
Is there anyone who has a solution for this problem?
to be continued...