Networking & wireless forum: Linksys WRT45G > Whole network gradually degrades

Alright, heres the setup:

(1) DSL Modem
(1) WRT54G Router
(1) Desktop WinXP Pro SP2 machine, hardwired to router
(2) WinXP Pro SP2 laptops, wireless - one with built-in wireless, one with a Linksys WPC11 Instant Wireless Network Adapter, version 3.
(1) WinXP Media Center SP2 laptop, wireless
(1) Ubuntu Personal Edition - "Feisty Fawn" 7.04 laptop, wireless

The router has been set up to use WPA-TKIP encryption on channel 6 with an always-on PPPoE connection; it also has an IP of http://192.168.2.1 to avoid an IP conflict with the DSL Modem. DHCP Server is enabled with default settings, MAC Address Clone is disabled, and Operating Mode is set to Gateway. Access is set to allow both B and G, as one of the WinXP Pro laptops is an older model Toshiba that only recognizes the B protocol (that's the one with the WPC11 card).

Here's the problem:

Initially, the setup works flawlessly. All computers are able to see the SSID, connect to the network, and connect to the internet. The connection is fine, doesn't slow down, and is overall pretty healthy. All computers are able to access http://192.168.2.1, and are prompted with the username/password prompt, can change settings, port forwarding, etc.

Now, wait a few days. Usually about a week.

First, the Ubuntu laptop starts to get spotty with it's connection to the internet. After about 10 minutes, the connection to the network starts to fail. Sometimes it will see the SSID, sometimes it won't - and it's not a matter of moving the laptop. It stays pretty much on the same desk and acts as a desktop 98% of the time. If you try to access http://192.168.2.1 nothing happens. No username/password prompt, no "access denied" message. Rebooting the machine doesn't improve the situation. Manually entering in the SSID and password don't work, either.

At this point, all of the other computers are still able to access the internet. However, trying to access http://192.168.2.1 with *any* of the other machines fails. Even if I try to access it with the wired desktop machine, there is no prompt, no error messages, just a blank white page with a "done" status in both Firefox and IE.

After about a day or two of this, all other machines except for the wired desktop computer will cease to see the SSID and all connections to the network fail. No network access, no internet access. The only machine still connected to the network and the internet is the wired desktop machine.

What I've tried:

At this point, I usually power cycle the Linksys router - and viola, back to normal.

As far as settings-side, I've tried changing just about every setting I can that wont break the system; I've tried changing channels, changing the encryption, changing the IP of the router... and nothing seems to fix the problem.

I've also looked at just about every forum, posting, help-reference, and how-to, and I can't seem to find a solution to the problem, or even what's really happening to cause it.

Normally, I wouldn't take issue with having to restart the router. It's not a big issue for me, personally. But I set up this network for someone else, and at the end of this month I am moving, so I would like to fix and resolve the issue so that they don't have to restart the router every week. They are not very tech-savvy people and I don't want to make things more complicated than they have to be. At the least it's a minor inconvenience, and at the most it's a disaster and a call every week from them asking how to fix it.