Apple’s wireless Airports and wireless cards in laptops and desktops work well most of time, but sometimes settings get glitched and other things happen, so if you’ve restarted your Airport, you might want to start from scratch with the Airport settings on your iMac.
The reason why is that your Mac “remembers” several different settings for your Airport and other wireless networks that it has connected up with, and if those settings go awry, they need to be deleted and rentered and that might solve the problem. It’s the first thing to try, anyway, before you try more complex things.
First, turn off your Airport in the menu bar, just to keep it from trying to reconnect during the following process. To start, go to your System Preferences (This is for 10.4 Tiger, as you indicated you were using) under the Apple menu. Then go to Network, and select Location. Do you have an location for your home network? if so, select it and delete it. Then go to the other dropdown selector called “Show” and select Airport. At the bottom of that box will be a button called “Options.” Click that and uncheck “Remember Wireless Networks.” Close that, and in the previous box click “Apply Now,” in the bottom right hand corner.
Now, in order to really clean out the old wireless passwords and configurations, you need to go to Keychain Access, which is a program in your Utilities folder, which is inside Applications. Keychain Access stores wireless passwords, other logins and passwords for web forms and programs, etc., so you don’t have to type everything in agan and again. When you’re asked if you want Safari to save a password for you, Keychain Access is where it is stored.
Start up Keychain Access and scroll through the list of networks, web forms, passwords and find all the instances of your network. You really need to find them all; leaving even one will not do a complete housecleaning. You don’t need to delete the passwords for other networks if they work OK, but be sure and get all of the passwords for your network. The reason why there might be more than one is that Keychain will even remember the wrong passwords, too.
Keychain Access might ask for your adminstrative password to delete a wi-fi network, and if you’re running in a less than administrative account, you’ll need to log out of that and into one with admin privledges
Now, when you’re done with all that, restart. Turn your Airport back on and find your network and enter your password when asked. You should be able to connect right away and stay connected.