I've seen this too, but only at work. At home I have a Netgear, and it works fine. At work, about every 3 hours the IM module starts chiming that it lost connection. Wifi is showing connected, but I can't connect. I drop wifi and bring it back and it's all fine.

Could this be a DHCP renewal issue? Maybe the router is expecting a lease renewal every so often, regardless of settings, and dropping it if it doesn't see it?