2011.5 iMac 27'' screen blackout. Gpu was changed by Apple a month ago and it still does this funny screen blackout in a loop. I force it to sleep either with the button in the back or the keyboard. Finally restart and it blacks out during the boot process.
It also happened when I tried to boot the diagnostics.
Tags: 2011.5 iMac 27'' screen blackout, iMac, 27'm, Black, Screen, Blackout, 2011