i have problem with wifi connection on my N900.
i tried with two different wire-less network.
but i have found same problem on my device like
its works fine when it get connected with wifi network.
but after some time if i don't use the internet than i won't be able to do internet again. so what i am doing now i have to disconnection the wifi connection and again i connect wifi connection than again works..

Probably because of PSM not supported on your routers

http://wiki.maemo.org/Wifi_power_saving_mode