I recently upgraded from a DNS-321 to a DNS-323 and although I don't have the same problem as the OP, I am having a problem with accessing the DNS-323 from a PC with Win7-Pro 64bit platform if I map a drive letter to it. (Though I'm not having a problem with it being reconnected on restart of Windows.)
When I launch any MS Office application (.doc, .docx, .ppt, .pptx, etc) there's a HUGE delay of 20-40 secs whenever an MS Office app reads from, or saves to, the mapped drive letter (including whenever I double-click of a file from Windows Explorer, for example). No such delay happens from other computers in my network running XP (any file type), and no problems from other applications that I've noticed (e.g. launching/writing PDFs, MP3s, AVIs, etc, are all OK). MS Office access to files on local drives including USB keys operate perfectly with no delay.
I've scoured the Internet and so far, the possible frontrunners in terms of solutions are:
1. removal of MS hot fix xxxx - which I don't have installed
2. disabling SMB2 - did this on my PC ... no impact (is there a way to do this on the DNS-323?)
3. some weirdness with DDE - doesn't seem to apply to Win7 (appreciate info on this if it does)
One VERY odd thing I found is that accessing files from "Network Places" works flawlessly ... only access through the mapped drive letter is a problem!!!

Thoughts/ideas?!?
TIA,
DH
EDIT: Forgot to say, I'm running MS Office 2007 right now but do have the option of upgrading to Office 2010; however, I don't want to play this card unless I have to.