Last week my brother in law came over with his PC (windows 7 x64, 15.11 drivers) which contains a Sapphire R290.

His system, his GPU
When we started his Windows 7 system, the CAC-1070 worked instanteneously at 4k60p:
[img]http://insights.club-3d.com/wp-content/uploads/2016/04/2.-Windows-7-1.jpg[/img]
This picture is his system with 15.11 drivers in Windows 7

To my understanding this rules out the possibility that my Samsung screen is the culprit.

Swap
Next, we decided to swap the videocards:

• My R390 into his Windows 7 system (15.11 drivers)
• His R290 into my Windows 10 system (16.4.1 drivers)

[img]http://insights.club-3d.com/wp-content/uploads/2016/04/1.-2016-04-10-12.29.46-1.jpg[/img]
Left his system with my AMD R390 Gigabyte GV-R939G1 GAMING-8GD
Right my system with his Sapphire R290

My system, his GPU
We started up my system with the R290 which resulted in success:
[img]http://insights.club-3d.com/wp-content/uploads/2016/04/3.-windows-10.jpg[/img]
This picture is my system with a R290 with 16.4.1 drivers in Windows 10.
To my understanding this rules out the possibility that Windows 10 is the issue in my case.

His system, my GPU
Next we were very curious of course wether or not my R390 would work in his system.
Alas, when we fired up the R390 in my brother in law’s system, the video-output was capped at 4K@30Hz.

Findings
To me it seems it must be my AMD R390 Gigabyte GV-R939G1 GAMING-8GD.
However I am still not quite sure it is hardware or software related. 
Could it be that the 8GB of VRAM plays a part in this?

I am looking forward to your advice.

Regards,

Erik