Ether-net LAN, Computer to Computer Wireless

Hi,
First post here, and could use some help.
For over a year with my PowerMac G5 connected to my LAN via ether-net I could establish a computer-to-computer wireless network between the G5 and my G4 laptop (this is not internet sharing) and happily surf the net from the G5 and transfer files back and forth between the two computers.
It has stopped working. After an exhaustive 2 hour 40 minute call with the Apple Product Specialist, Apple ducked and said this is not a supported configuration and had no idea why I was able to run it for a year.
In a nutshell, the G4 laptop is trying to bind to the network using the G5's ether-net address, not it's Airport address (how does it know the G5 ether-net address?). The only other anomoly I know of is that I downloaded a program on the G5 that crashes on launch.
How can I get back to my original configuration, i.e. G5 ether-net LAN and Airport computer-to-computer network with G4 laptop?
Thanks in advance.