I might have found a fix, but not sure how or why it fixed it.
I just purchased the DS411+ ii tonight, and was enabling jumbo frames on all my NICs.
Played around with DSM, and it would freeze up on me, and I couldn't log back into DSM.
Not only that, my write speeds were atrocious, maxed out at about 35 MB/s.
Also, test video files transferred to the DS wouldn't play.
Frustrated at all of this, I decided to just disable jumbo frames on all my NICs.
Restarted everything, and lo & behold, all my issues were fixed. I could log back into DSM through the web interface, and the test video files were playing perfectly.
Hope this helps someone else!