imac 27 late 2013 black screen until after several restarts

If you find your lights flicker (LED lights more so) you really need a backfill UPS, We were burning out power supplies until we went with UPS's for our desktop systems. Even after checking out the AC power wiring we're still not done! You need to get a good surge suppressor or better yet get a good UPS unit. Make sure the outlet is properly grounded (three prong) and the breaker panel is likewise properly grounded and the grounding point is likewise in good shape. Make sure you have a dedicated outlet (or one that is only supporting lights in addition). I would strongly recommend you get a qualified electrician to check your house wiring. This occurs when your Macs kernel, or operating system, detects an issue that. The cost will be lower than what you'll end up paying trying just to get the parts. The most common cause of a Mac continually restarting is called kernel panic. I want to perform an Apple Hardware Test/Apple Diagnostic on the iMac. Everything works except the screen on the iMac. It is possible for me to use the computer with an external monitor.

I would recommend taking your system into an Apple Store and letting them deal with this. My screen have gone black and with flickering blue and yellow lines both horizontal and vertical. Either the power supply or the logic board or both could have a problem! Often when I see this type of issue the system had encountered a power surge either from the power line or a lightning storm.

Imac 27 late 2013 black screen until after several restarts