Just a thought: The symptom you describe ("random black screen, lasting for 1-2
seconds") can be caused by bad HDMI cables - especially when using 4k / 60Hz
modes - I had experienced that in the past on two occasions.

Have you tried using a different (preferably short, high-quality) cable?

BTW: Only very few HDMI cable vendors guarantee their products to work up to
600MHz clock frequency, which you require for 4k 60Hz.