https://bugs.freedesktop.org/show_bug.cgi?id=102389
--- Comment #7 from dwagner jb5sgc1n.nya@20mm.eu --- 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.