If I understand your post correctly, the PS3 doesn't "see" your dns-323. Mine does this as well - I'm sure they all do from time to time, but a refresh of the UPnP server is not what you should be doing. It is likely that the HDDS are just hibernating.
When my PS3 doesn't see one of my 323's I do one of 4 things (in order from quickest to longest fix) :
1. "search for media server" in PS3 (quickest fix)
2. Go into PS3 network settings and do an internet connection test
3. On a pc, map a drive on the 323 and navigate through the folders to get the hard drives out of hibernate, and then do step 1.
4. Login to the 323 and reboot it. Then do step 1 (longest)
The only time you have to refresh UPnP server is if you add new media, or media playback isn't working/DLNA errors.