I'm having this same problem. My DSM-520 is hardwired. I have 3 servers: (1) my PC, (2) IOMEGA NAS, and (3) LaCIE NAS. Regardless of which server I'm connected to, it drops after a few minutes (playing music files), but once I select "OK" I can reselect from the "lost" server.
Makes the DSM-520 nothing more than an expensive paperweight. Driving me nuts!