One quick fix if the Keychain Access utility is not storing the wireless password: 1) turn off the AirPort card, 2) go to Network preferences, highlight the AirPort connection, click on Advanced, 3) in the AirPort tab, remove the network in question from your list of preferred networks, 4) open Keychain Access and delete the entry for the AirPort network password, 5) restart and then manually rejoin the network, tell it to remember the network.
As far as the connection dropping, there is no setting on in the Mac OS that would cause it to release the connection. If the computer goes to sleep and you wake it up, it could take 15-30 seconds to automatically reconnect to the wireless network but nothing else AFAIK.