Internet Connection Intermittently Drops Out

I'm a new MAC user, I have an IMAC running Tiger and using an Airport Extreme for my wireless connection. Recently I have been experiencing intermittent failures to connect to the Internet in which I need to use Network Diagnostics or restart the computer to restore. My Airport Utility in the Menu Bar indicates I have a solid connection (all bars in the black) and I have a green light on my Extreme. These are happening at intervals from every 5 minutes to 1 hour. I don't believe it's the Airport Extreme as none my wireless connections to my Windows Computers are affected. I have reset the Extreme to Factory defaults to no avail. It is not limited to Safari, but Firefox, Entourage etc. are also affected. If i leave Network Diagnostics on and get a lock up it states my Internet is working normally most of the time (occasionally has Internet and server as red), however running through the diagnostics normally restores it. The only commonality is when I go through the diagnostics, a different external available network from apparently one of my neighbors has been added or deleted. HELP!!!!!