I am having this same issue with NCM ver. 7.4. I have tried all of the suggestions to no avail. I have set the simultaneous uploads/downloads to 3. The job I am testing with is only 11 devices, I have broken them down into 2 smaller jobs but continue to have the issue. There are no other jobs running, no server backups, no unusual network activity, all high bandwidith links.
Any more ideas?