Dropped WiFi problem-- very puzzling.

A medial of honor goes to whomever can figure this one out...

My Mac Pro (OS 10.6. desktop drops WiFi connectivity intermittenly (no email, no web browsing; messages are usually "cannot connect to server" or "server timeout.") My iPhones and iBook G4 also experience the same problem on my home network. My WLAN is Airport card in the Mac Pro receiving from an N router, and cable modem.

Bonjour devices/applications, even when the network is running, often DON'T work.

This problem ONLY happens when security (WPA, WPA2, etc.) is turned ON at the router.

The only quick fix is turning Airport on and off again, or running Connection Doctor (which usually just says "Network Config has changed...")

What I've done already (aside from the usual restarting of everything, unplugging), etc:

- I've had this problem through two different modems, two different routers, two OS's (Tiger & now Snow Leop)
-Tried different SSIDs, router security, etc.
- Unplugged any cordless phones in the house
-Called the highest levels of Apple Support. Couldn't figure it out. Ditto for router company and cable provider.
-Downloaded a WiFi scanner app to check any interference (although don't really know how to use the app fully.)
- Ran every check possible on Network Utility and checked Network settings

This is very frustrating... only theories are authentication errors (maybe in Keychain?), Firewall (but where-- router or OSX?) any help appreciated. Can't figure this one out!