A: There are multiple things that could cause this behavior, most of which pertain to HDMI 2.0 requiring twice the bandwidth of HDMI 1.4, nearly 20Gbps.
* HDMI cables which only comply with the 1.4 version of the specification will not operate correctly at 4Kp60
* Many 4K displays have upgradable firmware. Updating the display firmware can often help HDMI 2.0 behavior
* Updating the graphics driver on the host system can often improve performance/stability at higher resolutions and refresh rates