Right, new information.
Have deleted all keychain entries pertaining to Wifi, and all Wifi networks from System Preferences.
iStumbler can see the network with only one other network on Channel 1, 4 on channel 6 and occasional networks on 11 and 13. Strength is 95 to 60% all around the house, however when the wifi is not working, the network is listed under the main list, but not under the Airport > Visible Networks list.
I can sometimes connect, but often have to do it manually and will eventually be dropped. Ethernet works and so does the web.
Here are the WiFi settings:
Wireless Channel: 1
Extension Channel: 5
SSID: Pelham123
Wireless Mode: 11b
gn (only option other than off)
Broadcast SSID: On
Protected Mode: Off
Bandwodth: 20 + 40 Mhz
QoS: Off
Security Mode: WPA/WPA2 Personal
Authenication: WPA-PSK + WPA2-PSK
Encryption: TKIP + AES
Password: 10 digit mixed
Guest Settings: Disabled
Help?