Web radio stutters with Hifiberry Digi+ - transformer

Hi all,

I’m a happy new user of Volumio on my RPi4. I have it configured with a HifiBerry Digi+ Transformer. The optical out goes to my Argon Dac1.

Most of my audio runs great (lossless/mp3 from USB Hard Disk). However, when listening to any web radio it starts to stutter after about a minute.
Now you may suggest that the WiFi is the problem, but I also tried connecting my DAC through USB and than it runs without any problems. (I do not want to do that in the final set-up however)

Do you have any suggestions to fix this issue?