Help! Airport "out of range" error message & problems connecting to internet

Hello all,

Thanks in advance for advice.

I'm an iBook G4 user. I've had my computer for about 4 years and never had any problems, but in the past two weeks, I've been experiencing a lot of difficulty with my internet. The problem's got to be with airport, because I'm connected right now with an ethernet cable and am having no issues.

Using my airport, I can still usually connect to the internet-- in fact, when I first connect to a wireless network, I have strong signal strength-- but I lose my internet connection after just a few minutes. When I click on "Open internet connect," I can often reconnect to the network, but then I get the error message "out of range of network" and am booted off again.

This has happened to me on two separate internet networks-- both in my home and at the library-- so I think the problem is with my computer, not the signal.

There's not any physical object blocking the signal as far as I can see, and I've been able to use this wireless network in the past without any problems. (It's my partner's network and is and password protected, so it's not like I'm stealing it from someone next door or something.)

I'm running Mac OS X.

I've looked at getting airport express, but I don't want to pay money for something that may or may not fix the problem.

Suggestions would be so appreciated... Thanks!