Macbook Keeps Forgetting My Wireless

Hey Everyone,
I have had my Macbook for about two weeks now and I am having a constant annoying problem. Every now and then when I open up my Macbook from sleep it has forgotten my wireless network. When I go into network settings my network is listed, but the computer still says it is not found. However, in the list of available networks that it gives me, it lists mine. I doubt it is supposed to be acting like this. It stays connected fine for a while, no matter if I reboot, sleep, it stays connected. But then at other random times it forgets and I need to reenter my key to connect to my network, even though the key is listed in my keychain access under system, therefore I have had to resort to having a text file with my key saved to my computer for when it forgets so I do not have to go looking for it.

Does anyone know what would be causing my Macbook to stop recognizing my network and making me have to reconnect and put in my WEP key?

Hope someone can help, thanks.

PS - It isn't a router that you need to put in the WEP key over and over because it works fine on my Windows XP dell. Also, if I turn off airport and turn it back on, it will then connect to my network fine. Just don't want to have to do this all the time.