I had my PS4 repaired because of this same issue couple of months ago. They told that no parts where changed, only system update were made. Now the same issue is present: picture is black and sound comes and goes, see the video. My opinion is that graphics card is broken but Sony repairs say that everything is ok. I have changed HDMI cabels and ports and tried with blu ray player that they work.
https://youtu.be/t2lbnR5BFwA
