There are several things to test before deciding the unit is defective:
1) Try a different cable going to the DNS-321.
2) Enable/disable the jumbo frames setting.
3) Try different drive(s) in the DNS-321, and/or just one drive.
4) Try the 3 different link speeds in LAN settings: auto, 100, 1000
5) Ensure all the various DNS-321 apps are disabled iTunes, FTP, etc.
If it still fails with these tested, then you can consider hardware problems.
1. I replaced the Cable, as well as bought yet another brand new Gigabit switch.
2. Cannot enable Jumbo frames as not all devices on the network are compatible.
3. I do not have any spare drives to test and currently have data on the RAID and would rather not break it (I would have to copy all the data off...slowly...then copy it back...slowly)
4. Tried all settings for link speed.
5. All apps are and have been disabled.
After replacing the cable and switch, the speed seems a LITTLE better, but I am still getting skipping when playing MP3's in iTunes (iTunes library is set to a mapped drive on the 321). When the songs skip I notice only one of the hard drive lights on the DNS-321 flashing. It will stop flashing and then the song resumes.
Another thing I noticed is that the web interface only freezes when I click on the iTunes server button, as if I were going to configure it.