iMac 27" Late 2015 Wake up Black screen

Hi guys,

I'm recently (few days) experiencing problem with my iMac when it suppose to wake up after entering standby power state. I went through many forums and discussions already, but all of them are old, outdated and none of them gives really solution to the issue.

Problem is related exactly to iMac (Retina 5K, 27-inch, Late 2015), 4 GHz Intel Core i7, 16 GB Memory, AMD Radeon R9 M395 2048 MB, 2 TB Fusion Drive. There's fresh installation of latest macOS Sierra 10.12.1 with all relevant updates.

When system just falls a sleep (or when I put it into the sleep manually) and then I try after few mins to wake it up everything works fine.

But after some hours of sleep, probably when it enters the standby power state and then I try to click anything mouse click or keyboard to wake the system up, I can see significant increase in power load on my UPS, can hear HDD spinning up and can see the back light coming from the screen, also mouse pointer is shown, but the screen remains all dark and black, just only I realized really small strip of 2 or 3 rows of the random colored pixels at the top of the screen (unfortunately I'm unable to make screenshot of it) and nothing more happens for next 10-20 secs. I have to move the mouse and click it few more times, then after system wakes up and logon screen becomes available.

I cannot identify exactly the cause of the problem, but this behavior started few days ago, I don't know if it could be related to my experiments with Boot Camp and Windows 10 installation, which I tried week ago. But there was nothing going wrong with Boot Camp Assistant and with Windows 10 installation either. Anyway, when I finished my experiment with Windows I just removed Windows 10 via Boot Camp Assistant from my HD and went back to Single Partition configuration I erased Macintosh HD with Disk Utility, I checked it passed all the First Aid checks with no errors and I did fresh new installation of macOS Sierra on it. Everything went well with no unexpected errors but now I'm experiencing this problem when system doesn't respond properly on wake up.

I did the standard trouble-shooting steps advised everywhere like reseting NVRAM and SMC, I did fsck -fy in Single User mode, I ran again First Aids from Disk Utility in Recovery mode. None of these helped.

Any advice or comments would be highly appreciated, many thanks in advance, regards...

Manolis