USB WiFi adapter that can replace iMac Airport

You tried all the solutions, including backing up your data, erasing the internal drive, and installing a fresh system on a blank volume?

Alternately, as a test, you can install a fresh system on an external drive and set it as the startup disk. If there is improvement, you can then back up your data, and do the same thing with your internal drive.

As for the USB wireless adapter, OWC is a Mac-centric company and they have one under their NewerTech brand.

http://eshop.macsales.com/item/Newer%20Technology/MXP802NU2C/

I have their older 802.11g adapter, and it works well.

Listen... a Mac is a computer like a Windows PC. It's hard drive can have data corruption too. And you may have even installed some third-party software that is causing this problem, such as a custom kernel extension. Or you may have inadvertently deleted or moved some key system component. There are many possibilities, but the solution of last resort (for any computer since personal computers had hard drives) has been to back up personal data, erase the hard drive, and re-install. If you go to the Genius Bar at an Apple Store, they'll do some trouble-shooting and if they can't figure it out with reasonable effort, they'll erase the drive and re-install the OS. If that does not resolve the issue, they'll conclude it is a hardware problem start investigating which component is faulty.

If Macs never failed or had issues related to its hard drive, Apple would not have created Time Machine so users can do ongoing backups without thinking about it.

I have no idea what you have tried, since you have did not specifically state it. You should have already tried these three basic steps

Run Disk Utility and use +Repair Disk Permissions+ on the +First Aid+ tab
Reset PRAM - http://support.apple.com/kb/HT1379
Reset SMC - http://support.apple.com/kb/HT1543

And before doing an erase and install, you should try one more thing as a test. Create a new admin user account in System Preferences Accounts pane

http://docs.info.apple.com/article.html?path=Mac/10.5/en/8235.html

Log out and log in to the new account. Try wireless networking while logged into this new account. Does it work better or is it the same?

If there is improvement, the cause is likely to be confined to your normal user account and not the overall system or hardware. It may be a corrupted user preferences setting or some third-party process that is running in the background. Be sure to check System Preferences Accounts pane +Login Items+ tab (in your normal account) and review the processes that start up automatically at login.

If there is no change, the cause is likely to be related to the overall system installation or to hardware. Backing up your data, erasing the hard drive, and re-installing will serve to either resolve the problem, or rule out a software-related cause (pointing to a hardware-related cause).

If you have questions about doing an erase and re-install, please post back.

Oh, and for this particular problem, you should also investigate the possibility that the cause is at the wireless router end of the connection.

I solved my constant wireless (no) connection problems by buying a new N router and, like magic, no more problems. I guess the G wasn't far reaching enough.