So your ability to connect is completely random... Your card could
be installed incorrectly - it wouldn't be a bad idea to try re-installing it. If you have the older, white, AirPort card, make sure that the card is parallel to the pins, and that the antenna connector is pushed in all the way. Click here
for Apple's instructions. If you have an AirPort Extreme card (it's about half the length of a credit card), the instructions can be located here
(12" PowerBook G4) or here
(15" PowerBook G4). There wasn't a guide for the 17" PowerBook, but the procedure is basically the same.