How do I know if my Airport Extreme Card is going bad?

I have 3 computers on a home network. I have the router (Airport Extreme) in my guest house and it is working properly. The tenant is able to get her email and browse just fine. I also have a Airport Express in the front house that extends the signal via WDS. I have a 24 inch Intel iMac 2.4 that is connected to the network and runs fine. When I check in the Airport utility the iMac can see both routers. I have a Power Mac G5 2.0 that has a Airport Extreme card in my studio which is behind my garage. It shows that it is connected to the network and at full strength but I cannot browse or get mail. I tried to run diagnostics but when I do it does not find my network. I also cannot find either one of the routers from the Airport Utility even though the Mac is about one room away from the main router. I tried renewing the DHCP. I also even uninstalled and reinstalled the Airport Extreme Card with no luck. I can see my network from the Status Bar where the Airport Signal Strength is but again...cannot find the routers in Airport Utility nor find the network within the Networking Diagnostics.

Any advise?