It'd be best to use the LiveCD from your current implementation so it's a more legitimate comparison. As you point out, the wi-fi driver isn't built-in, so it never fires up.
I found my old Edgy 6.10 CD and booted 5 times from that.
All pretty slow (4min 6sec to 4min 16sec) but consistent and no obvious hang-up or failure.
I never did have wi-fi from that CD though, so not sure what that proves?
Coupled with your favorable memtest results (yes, it does run forever it would seem to not be a problem with the computer at large, but be a problem with either your wi-fi hardware or the software that runs it.
Really? Where does it fail; on detecting available networks or on requesting an IP address from the DHCP server? Does it consistently do either?
In any case, I cannot get wi-fi connected if I bring the router on after booting.
Regardless, I think the prognosis is the same; download the Hardy LiveCD and see how that works for you. If it works, you could perform a Distribution Upgrade from the upgrade manager, (backing up your data beforehand just to be safe). You'd probably want to do a clean install just to make sure the problem didn't carry over.
If the software 'solution' of the Hardy LiveCD doesn't work consistently, then it's probably a hardware problem with the wi-fi card or router; which means firmware upgrade or replacement.
My guess would be that you've run an 'apt-get upgrade' at some point that upgraded one of the networking packages and now when your wi-fi card errors out it's not trapping the error properly and is causing the NFS daemon to hang. Total speculation, but you may want to check the bug files on launchpad.
I then went back to "normal" and recorded some bootcharts (I have hundreds...) of a good boot (gutsy-20080520-3.png) and a failure with hanging at "Configuring Network Interfaces" then sticking at "Starting NFS kernel daemon" which needed switching off (gutsy-20080520-2.png).
Do these provide any clues?