ECF,
In my case, I am trying to watch a 720p .mpg file sourced from an ATSC tuner card.  The file was captured in raw transport stream format and then stripped down to a 720p video stream and 5.1 audio stream.  The file on the DNS-323 is 
\\media1\Volume_1\Media\RecordedTV\recording.mpg 6,611,580KB.  All devices are hardwired to a DGS-2208 switch.  I can view this same file in several ways, the first of which does not work completely and the rest work fine.
1) DSM-510 -> UPnP on DNS-323
       This method does not work. The File plays for about 20 minutes and halts.
       DSM-510 shows the following when selecting file and hitting info button:
   TITLE: recording.mpg
   DATE: 2008/02/29 16:37:25
   SERVER: Media1
                MIME TYPE: video/mpeg
                SIZE: 2.3GB
(Notice that this size is 6.3GB-4GB=2.3GB.  32bit boundry???)
2) DSM-510 -> Nero7 UPnP server on PC ->  M: mapped to \\media1\Volume_1 on DNS-323
       DSM-510 shows the following when selecting file and hitting info button:
   TITLE: recording.mpg
   ALBUM: RecordedTV
   DURATION: 01:05:43
   SERVER: MediaHome
   MIME TYPE: video/mpeg
   RESOLUTION: 1280x720
   SIZE: 6.3GB
           File plays correctly
       
3) PC -> UNC on DNS-323
       PC shows file as 6,611,580 KB Movie File (mpeg) and plays correctly
etc....
I have tried doing a manual update of the UPnP server on the DNS-323 (shows "Refreshed Successfully") as well as letting the automatic update run. 
DNS-323 is running 1.04 firmware with two disks in Raid-1.
UPnP AV Server   *Enable
Folder Volume_1/Media
Auto Refresh    [1 Day]
Take another file that shows 5.9GB from Nero and it shows 1.9GB from the DNS-323 UPnP server.  Looks like a classic case of a 32-bit error.
Regards,
Karl
PS: The spell checker wouldn't let me use the word "wait" with an "ing" on the end.  It kept replacing it with [Explictive removed]