wifi signal good, internet sporadic

I recently installed an ATT (G) wifi router. I have a laptop and desktop running off wifi. The first few days everything was fine. Then, although signal strength is always strong, browsing became sporadic. I tried various changes (firewall, encryption), but nothing helped. At this point I can't surf with the laptop (unless I use a LAN cable rather than wifi - then no problem), and the desktop will surf every few days (again, always a strong signal though). My only wireless devices are 900mhz phones, but unplugging them didn't matter.
So why would I be able to surf some times and not others?

 

May be this can Help.

http://www.ezlan.net/wbars.html

Link to: Wireless - Basic Configuration.

Link to: Wireless Security.