David,
I think I mentioned this one already, but have you set IPv6 to "off" (System Preferences -> Network -> Airport -> Advanced -> TCP/IP -> Configure IPv6 "off"), if not, try it.
What is your network setup, as in what router/modem, what is providing wireless access, what is it set to b/g/n? Is it acting as a DHCP server. Are you extending the wireless network, if so, with what are you doing this.
In the airport advanced options, go into AppleTalk, make sure "Make AppleTalk Active" is disabled. Also go into "Ethernet" and check what it's set to, ideally configure should be set to "Automatically"
Part of me is tempted to say to replace whatever you have with Airport versions, perhaps you could borrow some, rather than purchase outright. I know, additional expense that should not be required, but if it did fix the issue then we'd know there is some difference between things, and perhaps Apple is being naughty.
I'd love to help you get this one sorted out, as I hate niggling issues like this.