Interference with my Express and Extreme?

Question

I have an Airport Extreme purchased several years ago to connect two iMacs. We have updated each computer, one is a newer iMac and the other is a MacBookPro (06). We have had trouble getting a signal to reach both computers since we first bought the Extreme. I added an Express and it seemed to solve the problem.
The Extreme is located next to the desktop computer. My laptop (which is never further than 40 feet), randomly gets dropped offline. I have to go through choosing airport, my network, etc, each time I'm dropped and each time I boot up. I never had to do this before. The only change I made recently was password protecting my network. I have done software updates since the problem started and also have run firmware and software diagnostic tests which were fine.
I've been to the Genius Bar three times, I've spoken with Apple Tech support for over 3 hours total on this issue. The Extreme tested fine in the Apple Store, I purchased a new Express and the problem is worse. When I get connected it will sometimes work 20 minutes, an hour, or 5 minutes. When this happens I have to sit right next to the Extreme to pick up a signal.
We have single wall construction, no cordless phones and I can't think of any other interference. Sorry this is so long but I'm desperate! Thank you

Answer 1

{quote:title=alohajam-Hawaii wrote:The only change I made recently was password protecting my network.}{quote}
Hmmm. That can make a difference, but I would think that should be exhibited as a performance hit as opposed to a reliability problem. Early AirPort Extremes took a hit with WEP (according to http://www.smallnetbuilder.com/content/view/30664/228/). After looking at iStumbler, there's an old (2004) but good article at http://www.smallnetbuilder.com/content/view/24311/98/ covering a/b/g nets; for instance, make sure your SSID is unique. Also, if running 802.11n, take a look at this article at http://www.smallnetbuilder.com/content/view/30664/228/ (esp. Fixes #4 and #5).
I found running 802.11n to be futile around here (well, it is Silicon Valley). [iStumbler] will give you a clue about things in your area, but I found that the local WiFi environment changed day-to-day as new nets came online, or folks changed the bands they're using and/or switched to using wide channels (channel bonding). I could never stabilize things for more than a day or two here, so ended up buying a new Extreme and running Radio Mode = 5 GHz only - the only one in the area (at the time). That helped lots.