After a few months (!) with irritation because the sound system cuts sound for a second or two after approx 10 seconds normal sound and CD, DAB etc turning grey I found an extremely simple solution.

I plugged in a Bluetooth receiver a few months ago. I used it to play Spotify in the car. Then I removed it and forgot about it. And the problems started to show.

Today I plugged in a 12 volt USB cigarette adapter and voila. The sound is now constant. Problem solved.

Before this the whole system with fibre cables was checked by the workshop without finding anything wrong.

I read about a similar problem caused by an aux in plug. It seems using such plugs may cause the intermittance problem.