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.

UPDATE:

I just noticed that it usually does this when coming out of sleep, however if I turn airport off and then turn it back on it connects to my network fine. Strange problem, but I think may have found a temporary fix.

Hey Everyone,
Thanks for the great answers. I tried changing the security to WPA and it seems to be working better. However, I have been trying to go into my keychain, and it allows me to get into my login keychain, but my password is not working to get into my system keychain. I am the only user on the computer, anyone know why I cannot access my system keychain to get rid of the logins?

Thanks

UPDATE : Nevermind, strange, it is letting me in now :S