Go into the Hardware Settings page of the Delta Control Panel. Change the Multi-track mode from "Single and In-sync" to "Independent."

When the Delta is set to Single and In-sync, your computer may be struggling to keep all channels synchronized. Set to Independent, your S/PDIF output is on its own and doesn’t need to drag the other outputs along with it to stay in sync. Transferring an S/PDIF signal is going to be less forgiving, as any sync problem that occurs will cause your receiving device (the DAT) to exit record mode.