Imac won't fully boot
Imac (mid 2011) is stuck in the endless boot cycle but will not fully boot. After the progress bar completes the screen goes to grey and then black and starts over again and again. I've tried setting the PRAM and completely pulling the power. The first clue was a couple of days ago when during normal use (on YouTube) the screen just suddenly went black and everything shut down automatically. Then after rebooting the machine would only last about 10-15 minutes before going black again. And now, it won't even get through the restart cycle without this endless loop of auto-rebooting.
iMac 27", macOS 10.13