Airport doesn't remember wireless network

Airport won't remember the wireless connection after sleep or shutdown. It keeps reporting that the WEP password is incorrect, so I have to use the diagnostics button to establish the connection each session. The ibook remembers with other networks, just not this one to the Zoom wireless modem. Also, to establish the connection, I have to select WEP 40/128 although the router shows WEP 64. Any suggestions on how to get the mac to remember and or set it as a preferred network? TIA, felipe.