My first thought is that somehow or another the airport card is loose inside the unit. So I would start by opening it up and making sure it's in the slot properly, and that the antenna cables are connected properly.
I'm also pretty sure there are no 11n cards for the G4 lineup, period. 11g maybe, but certainly not 11n. So if that's some kind of third party card, that could also be a contributing factor.
I have a Power Mac G4, dual 1.25GHz, OS 10.5.8, wifi card installed, Airport Extreme 802.11n (1st gen) v.7.6.1.
I am encountering Airport recognition problems periodically. Sometimes the machine does not see Airport after powering up, sometimes after an Internet session, or after the machine has been inactive/asleep. In other words I cannot go on the Internet. I usually restart, go to Airport utility to verify connection, which it sometimes does show a connection, and then able to access Internet. It doesn't matter what browser used. The problem occurs with any browser. Any remedies? Please help thanks