why is it...

[sunnycalifornia]

that sometimes I can connect to a wireless network with excellent signal strength but not use the internet? For example, sometimes I can sit in my living room and steal wireless from people in the other dorms in my hall and the internet is really fast, but then othertimes I can connect to the wireless network like I always do but I don't actually get internet (Firefox shows up failed to load page, AIM doesn't work, basically anything that requires internet, etc.)

I try to repair and disable/enable and restart but nothing works.

Suggestions?