iMac 5K goes to sleep mode (blank/black screen), the screen will not wake up unless Imac is rebooted

Got this bug just last week awhile after updating to El Capitan......

https://macperformanceguide.com/blog-2016-01.html#20160121_0928-OSX_10_11_3-Slee pBug-iMac5K

Model Name: iMac 27"

Model Identifier: iMac12,2

Processor Name: Intel Core i5

Processor Speed: 2.7 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 6 MB

Memory: 8 GB

Boot ROM Version: IM121.0047.B23

SMC Version (system): 1.72f2

Serial Number (system): C02GCEGXDHJP

Hardware UUID: E7A93B22-3482-5D4B-937B-B3EFC7110EFC

Had it dropped off at an Apple Authorized service depot after trying fruitlessly to solve the problem, was sure it was hardware related, backlight, video card etc. The no wake symptom occurred once at service and was fine after that until I got it back home.

The problem then returned repeated just after a few minutes of use and not even completing a reboot to the login, rebooting into internet recovering the screen was stable but that would only allow reinstalling LION. The screen was also stable after several attempts trying to get to recovery mode and reinstall of 10.11.6 which was successful, hoping that had solved the problem.

Currently at at 10.11.6.... unfortunately the problem has returned but so far much less frequently, lowering the brightness with the F1 key caused it twice to go into the black screen requiring a reboot, other than that it has been relatively stable but the bug continues to exist none the less.

Regards

Robert