Have you made a custom location for your network? By default, Mac's network locations are set to automatic. If you have not configured it, your Mac will be looking for access over all ports - Airport, ethernet, bluetooth, modem and firewire.
I would suggest going into your network control pane in system preferences and make a new location. Enter the relevent details, in your case Airport and set it to join a specific network when in that network location.
My PowerBook roams between home and work using a combination of 2 Airport networks, a modem and an ethernet network. Having individual locations set up make it easy - just select where you are and then you are online.
Sometimes my network drops out as well but all I do is switch to another location and back to the original one and it re-connects. It happens about once a month.
aussie_geek