thank you for your help so far.
1. I used the HifiBerry – Image
2. No additional devices, just the raspi, wifi-connected
3. Power supply is Rydges SpeedPower High-Quality supply 5V 2,5A
4. Luckily the problem did not occure since the last 5 or 6 days. No I´m running the Squeezebox server on another raspi cable attached to my network.
I did not install the Squeezebox server on the faulty raspi with the hifiberry.
5. No command line options set.
Maybe the sound remains stable (hope so). Troubleshooting remains difficult but I don´t think it is a problem of unreliable hardware.