When you enter the passkey make sure to have it save to your keychain, there should be a box above or below where you enter it that will say something like have the computer remember this password.
Also do you have multiple networks setup to your computer? If so, remove as many as you can. You can do this by going into your system preferences and then going to the network option in that menu.
While you are are in the network menu, check to see what your IP address is and what kind of authentication method is being used. Don't post your IP but check to see if after it drops you if its giving you a 169.xxx.xxx.xxx number.
Also are you running Mac OSx 10.7, 10.6, 10.5, 10.4?