Thank you for your reply. Yes, I have installed that latest firmware. As far as I know, it only fixes some bugs of a certain online service.
What puzzles me is that, before buying the DNS-320, I used to stream movies from a WD My Book external drive connected through a USB cable to the router, the router connected wirelessly to the WDTV, and the movies played fine, without stuttering. So I cannot understand why the movies now stutter when I connect the DNS-320 through a cat6 cable to the WDTV (through the router, not directly), since the wired 1 Gbit connection should be faster than the wireless n.
So I guess it is some bug related to samba between the DNS-320 and the WDTV. Could it be possible? Because, I repeat, the movies do not stutter when I choose media server. The only problem is that I cannot see subtitles in media server.