Ethernet AND wifi suddenly stopped working

After working normally for years my Powermac G5 running 10.5.8 suddenly stopped connecting to the internet, both wirelessly and via ethernet. The hardware is not the problem as another fresh instal boot patrition on the same machine works OK, and other devices connect to the router both via cable and wifi.

I'm totally baffled as to why this would suddenly affect both means of connection at the same time. I've spent literally days trawling these forums and elsewhere on the web and tried every suggested fix, all without success (scrapping preferences, setting up new locations, etc). Although I get the "self-assigned IP address" messages for both ethernet and wireless in the network configuration occasionally, none of the fixes for this issue have worked.

I'm really at the end of my tether. Migration Assistant doesn't properly restore the settings and authorisations for the dozens of applications on the original boot drive to a fresh instal - trying to resolve that would take more days (and possibly some expense) so isn't really an option.

All ideas gratefully anticipated!