I remember that behaviour on my wl-500w with the asus firmware, however it wasn't connected to FTP, as far as I remember.
Since I am using the google-code firmware,
http://code.google.com/p/wl500g/
I havent had those problems again.
You have to know if you have a wl-500gP v1 or v2. From there it is as easy as a common firmware update, if you dont want to run extra services.
You just have to reset to factory defaults before and after flashing the new firmware!