Please help: Chronic Airtunes Drop-outs/2.4 Ghz Interference

For several months, I have been battling interference with airtunes/AX, which is now so bad that it's barely usable. The music stops/starts constantly, and sometimes iTunes just stops. The signal strength indicator (normally 4 bars) jumps all over the place, going down to 3 and 2 bars. I used the same equipment set-up for over a year with excellent results until several months ago.

Here's the setup:

Airport Extreme connected to DSL router; Firmware: 5.7
Airport Express1 connected to stereo; Firmware: 6.3
Airport Express2 connected to USB printer; Firmware: 6.3
The AXes are set up to join the Airport Extreme network

3 Macs are on the network, and all experience the same symptoms. Powerbook1 (PB1) runs 802.11g; PB2 and iMac run 802.11b. PB1 has Interference Robustness enabled; the others don't support it. It seems to help slightly. No microwave or other Apple-defined interference source near any of the airport base stations.

PB1 is 24 feet from Airport Extreme and 20 feet from AX1
PB2 and iMac comparable distances.

I've been at the Genius Bar and have tried these things at their suggestion, and none have made a perceived difference.

-Turned off IPv6
-In Airport Admin Util:
--Changed channel from "automatic" to 1, 5, 9, 11 and others
--Increased multicast rate to 11 and 5.5
--Transmitter power 100%
--Interference Robustness enabled here

Thanks to Joe for suggesting Air Traffic Control widget to check your signal strength.
http://www.apple.com/downloads/dashboard/networking_security/airtrafficcontrol.h tml

It shows 2 neighbors on channels 1 and 6, so I put ours on 9. I hypothesize that it's a 2.4 ghz phone or other device.

What more can I do?

Thanks in advance for your help.

PB G4 17 inch; TiBook; iMac Mac OS X (10.4.8) 1 gig RAM