Well, at least you narrowed it down some. Small victories are nice.
Is your NAS tied into your system using WiFi? One thing you could try, is possible, try to move MAS and HTPC closer to the router and see what happens. There's definitely a possiblity you're getting radio interference. And all kinds of electronics can give it too; not just stuff that is wireless. In the kitchen, microwaves are like a level three tornado. Refrigerators give out, but they're usually like a small breeze. I had this issue one time with a VOIP adapter. It has zero WFi build into it. But is installed and setup about a foot from my wireless access points. This is what my LAN looked like at the time. It's the small gray box in the middle hanging upside down.
My router had done out (the PC to the right) and had to do temp setup. When I got the router fixed, and reset everything back up, the wreless access point closest to VOIP wouldn't connect to anything wirelessly. I found if I moved it, things would connect wirelessly, which lead to this setup:
Notice the VOIP is the far left. It was as far as I could get the ethernet cable to reach. It worked, kind of. the WiFi devices that access point,would loose their connection. What it end up being? It was the having the DHCP on in the VOIP adapter. I had used the VOIP as a router during the temp LAN setup. Apparently, whatever hardware in the VOIP adapter that the DHCP uses gave off enough radio interference to make at least one of my wireless access points completely useless. It only took me about a week with help of people way more knowledgeable about network to figure that out, and I stumble and guess on the solution.
If possible, try to eliminate everything you can hardware wise before trying to tackle the radio interference issue. There are just all kinds of variables that come into play, and unfortunately, not all of them may be obvious. But hey, you get, maybe, an interesting story with crazy pictures. Don't worry, my LAN setup is still just as nutty.
click the pic for a really big pic of it.
On a side note, this is what my office before I did all that. I started the clean up process around August 2007