I have had the exact same problem with DM-600 but I managed to fix/avoid it, all playback problem on DMS-600 always comes back to network problem. With DMS-600 if your network is working then playback will be working like a champ, but if you have network problem then nothing will work. The only fix that always 100% working is by the IR remote control to go to network and press "CONNECT" again until successful, make sure you can see that your re connecting attempt is successful. Cary network card is so bad and often drop with no connection even when the status is showing "CONNECTED", basically DMS-600 network card is so idiotic it keeps on dropping connection without itself realizing it (still showing connected on network status). I also find out that WiFi is a lot more reliable than Ethernet cable on DMS-600, I have the worst dropout rate on Ethernet compared to WiFi. If your router is able to identify network card, you can see that DMS-600 is using very cheap Chinese no name brand network card with Penguin logo on it, yes it is that bad. However, if you are already familiar and make routine of the procedure above, the playback will improve a lot to about 1-2 dropout in the span of 24 hrs.
Best permanent solution is off course to throw the dang thing into the trash can and buy something else better and never consider Cary anymore. Back in its glory days I used to be a big fan owning multiple Cary products including the infamous 805C monoblock, unfortunately Cary is a second or even third tier product nowadays after the departure of Dennis Had, I am still smacking my head for choosing DMS-600 over Bartok just because of my "expired" fond memories of Cary brand, DMS-600 has the worst interface and is the king of playback dropout compared to Mola Mola, Lumin, dCS and I have tried them all at home audition, fortunately I have Taiko Extreme on order and very glad I’ll be getting rid of this nightmare product.