D-link 2640

i've installed a d-link 2640 adsl router. both the wireless side and the dsl side seem to be working ok - if i connect via wireless link i get assigned an IP address, and the connection to the DSL provider works (i can browse the net, connect to email servers etc).

but if i enable WAP security i have a problem. i get connected briefly - can connect to the ISP for a while (less than a minute) then the "wireless meter" shows 0% and i lose the connection. This doesn't happen with security disabled.

I have this same problem with various clients (Ubuntu Gutsy/Win XP/Nokia E61) so i am (reasonably) sure the problem is with the router setup (or the router) and not on the client side.

any idea what the problem could be?

thanks for your help