W7 to NAS access hangs unpredictably

About six months ago, we bought a D-Link DNS-320 ShareCentre, installed two 1TB Seagate HDs, and began using it for our fileshare. Access and speed was OK, but not great.

Since about two weeks ago, our access to the fileshare has been intermittently very slow. Files (any size, 1 KB up to megabytes) load after a long hang-time, or don't load at all. The delay is far, far beyond the difference between 10/100/1000 bps connections. Navigating between folders, saving, moving, copying, and deleting files also have this intermittent hanging.

Our network:
[#] description
[1] TP-Link TL-WR841N, operating LAN only
[2] D-Link DNS-320, firmware v 2.03
[3] computer running Windows 7 Professional 64bit, on-board Ethernet connected to router
[4] computer running Windows XP

Trials with network configurations:
A) [1] connecting [2], [3], and [4]: both [3] and [4] experience problems connecting to [2]
B) [1] connecting [2] and [3] only ([4] disconnected): [3] still experiences problems
C) [1] connecting [2] and [4] only ([3] disconnected): no problems
D) [2] directly connected to [3] via Ethernet: [3] still experiences problems
E) [2] directly connected to [4] via Ethernet: no problems

Further trials with configuration (A):
i) Configuration (A) above, booting [3] in Windows XP (from separate HD): no problems
ii) Configuration (A), booting [3] in Windows 7 Safe Mode with Networking: no problems

More trials:
- I'm running Comodo Firewall on [3]. The problem persists when I disable it.
- Both [1] and [2] were on old (2010) firmware, and are now running on up-to-date firmware from the respective manufacturers.
- Ran Malwarebytes scan on my computer, nothing found.
- Ran SpyBot and let it have its way with all the things it found. Still have problems.
- Booting and NOT running MS Outlook just now, no problems...
- Opened MS Outlook, problems start.
- Closed MS Outlook... problems persist... end the Outlook task in task manager, no problems... could this be it?

It's hard to say "no problems" definitively, because the problem itself is intermittent. This is headaches.

Do you have any ideas? Potential root causes, or next-steps in troubleshooting? I'm loathe to format and reinstall my programs, but it might have to come to that.

Thank you for taking the time to read my plea.

you tried changing over the ethernet cable?

I had an issue with my nas drive and it turned out to be a faulty ethernet cable?

worth a try...

also what router you using please?

ascomputers, thanks for the tip. The problem persists when I swap out the ethernet cable.
The router is a TP-Link TL-WR841N.

It seems like Outlook is correlated to the problem. When either of OUTLOOK.EXE *32 or WINWORD.EXE *32 were running, problems. When neither of those processes were running, no problems.

I've moved a copy of all my .pst archive files to my local disk. So far, so good...