Mac Mini *half* loses all network connections

Question

Cheers folks, new one on me.  I have a Mac Mini 2.26Ghz Core 2 Duo. It's running a fully-updated 10.6.8.  It's connected via ethernet to an Airport Extreme which is extending an 802.11N network from another Airport Extreme elsewhere which is connected to the router.  The network is set with IPv6 *off* and is configured to run over Ethernet normally (this is because a wired-only NAS is connected to the same Airport Extreme as the Mini).
Here's the symptom, and it's an odd one for me:   Every couple of days, the entirety of the Mac OS X layer (not the BSD layer) will lose network connectivity.  More specifically:
No web browser will work ('Not connected to the Internet').
Home Sharing, served on this Mini, will cease to be visible from elsewhere in the house (AppleTVs lose it).
No internal services can be reached by name, either DNS or BonJour.
The App Store says 'can't connect.'
iTunes will say it can't connect to the ITMS.
This is true on Ethernet *or* on Airport (the Mini usually runs on ethernet alone with AirPort off; bringing up Airport once this has occurred doesn't seem to help anything).
Running the Airport Utility, it sees *no devices* (there are 4 on this network)
BUT!
If I go into the Terminal, I can not only get clear network traffic anywhere I want at the TCP layer, DNS seems to work as well.  So the following is true also:
'ping cisco.com' (or any valid DNS name) works fine - resolves DNS, pings.
'traceroute cisco.com' works fine - resolves DNS, traceroutes out across the internet, no problems.
'ping <IP address of NAS>' - works fine.
'dig <name>' - works fine.
pinging the gateway (the router airport) works fine.
Also:
Other Macs in the house (on wired OR wireless, connected to any of the AirPorts including the same one as the Mini) work fine.
Other Macs can see the NAS connected to that same AirPort fine.
iOS devices in the house connected to any AirPort work fine.
netstat -r shows what look to be proper routes - i.e. the default gateway is the router address (10.0.1.1) and the route to 10.0.1.1 is shown to be the MAC address of the router's Ethernet interface (anyway, as I mentioned above, network stuff from the shell works fine<!?!??!
Steps tried:
Turning off ipv6 - nope.
Applying all firmware/software updates (OS X 10.6.8) - nope.
Tried manually setting DNS to Google public DNS servers - nope.  Those work fine, and are still set when the network drops out, and then don't work, nor does setting new DNS servers in the Network prefpane.
Logging out/back in - nope.
Restarting the computer:  YES.  This works, all three times this has happened (all this week).  Serious PITA.
I tried going through the system log, but I don't see anything notable that changes or is logged when the network connection drops.
Every time the connection has stopped working, the computer *has* slept at some point since it was last known working, but if I sleep the computer manually, everything seems fine.
.....waaaaaaaah!  Help, this one is making me crazy. :-)  Thanks.  I'm terminal-savvy, and fairly OS X savvy.

Answer 1

I still have this issue.  About every one hour - two days the DNS layer will stop working and only a restart will fix it.
Killing mDNSResponder only resolves the issue sometimes and even then for a few seconds.
I have now found and created the following bash script, to see if this will be a more reliable non-restart fix that killing mDNSResponder in the terminal:
#!/bin/bash
#Script to unload and reload mDNS as it is a bit crappy
sudo launchctl unload /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist
echo "mDNS unloaded"
sleep 5
sudo launchctl load /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist
echo "mDNS loaded"
sleep 5
echo "Hopefully you can get on with browsing again"
I saved the above as mdns_reload.command, and then in Terminal ran "chmod u+x mdns_reload.command".  This makes the script executable.
The next time the problem occurs I am going to test how good this script is as a resolution.
The second time the problem occurs I am going to try the following, from a different thread from 2011:
I don't know if anyone will be helped by this, but the Macbook problems connecting to the internet are directly related to DNS issues. I discovered this after an upgrade to the OS X Lion preview.
The symptoms are as follows: a macbook (pro, mini, etc) can get an IP address from a DHCP server on any given network, but can not connect to the internet, even though other computers on the same network have no issues. If this sounds familiar, read on.
The problem is not related to 3rd party products, routers, Airport settings, locations or public vs private DNS servers, and you don't need to reset SMC or PRAM.
You will need to use terminal and you DO NOT want to use disk utility to repair permissions.
The most direct way to fix this is to copy the com.apple.mDNSResponder.plist and com.apple.mDNSResponderHelper.plist files from an internet connected Mac into your /System/Library/LaunchDaemons directory.
(You should know that it probably doesn't matter what version of OS X you have or the other Mac has, you just need the files.)
Next you will need to change ownership of the files just copied (ex: chown root:wheel /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist and chown root:wheel /System/Library/LaunchDaemons/com.apple.mDNSResponderHelper.plist)
Then, you will need to set correct permissions (ex: chmod -R 755 /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist and chmod -R 755 /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist)
The last thing you'll want to do is to verify that the ownership of the files copied is set to system (and maybe wheel) using "Get Info." (Conversely, you can use ls -l /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist to verify owner and permissions.)
You should not have to restart to connect immediately to the internet.