After a fruitless testing phase that included re-locating basestations, disabling Wi-Fi enabled non-Macs (like TiVo and a Wii), and removing an Airport Express from the network, Ged had a break in the case.
By using the freeware utility AP Grapher, he identified the wireless routers in his neighborhood. Even though several of them were hundreds of feet away from his house, he was still picking up their signals. At that time, the channel would be cluttered with radio noise, prompting the basestation to cease broadcasting until the noise went away. That could take a few minutes, or a very long time.
With his Mac's Airport Preferences set to Automatic, his machine would occasionally select those channels, so that was the first thing to eliminate. Most people use their router at factory settings, and most routers use channel 6 out of the box. By avoiding 6 (as well as one channel above and below), Ged has happily avoided the issue since Christmas.
Thanks, Ged! Here's hoping this tip helps you, too. If cleaning up your channel choices doesn't improve your wireless life, you might try the suggestions of some commenters on Mike's recent 10.5.2. wireless issues post and turn of "Delayed ACK" on your wireless connection. You can use the command in the comment to turn off Delayed ACK temporarily, or use Systemsboy's startup item to disable it longer-term.
Don't forget, our Sunday night talkcast is all about Mac OS X issues and problems this week -- don't forget to chime in with your ongoing aggravations!