Sounds like you may have a problem with an attacker executing a man in the middle attack. I dont completely understand what is going on, but that is my best guess... The only good way to prevent man in the middle is buy encrypting the transmission between you and the ISP. I know its already been said, but in my opinion it is something that can not be stressed enough. Also, I happen to have written a few articiles on WEP and WPA... and beleive me,
WPA is not signifigantly better, just use 128 bit WEP its better than nothing. And if you really want to be secure, I threw a perl script together a while back for dynamic WEP keys... it only worked for Adhoc though. Hope this helped...

-Shell_Coder