Update for some suggestions people have offered.
1) Disable Bluetooth
Tried that - no difference.
2) Set IP6 to local-link only.
Already had that setting.
3) Run with Wireless Diagnostics.
Was doing that after the problems were encountered. Made no difference.
Seems really weird to loose WAN but not LAN because in this case would be a router problem.
You may still connected to the network and yet have no access to any device on LAN.
I have exactly the same problem as mentaluproar but I tried 4 different routers and the results were identical.
Also, I confirmed that LAN connectivity remained throughout the temporary loss of WAN (with the OS X diagnostics tool running in the background and by manual pings, etc.).
The loss of WAN only would normally suggest a problem with the head end (Cablemodem or ISP) but in my case the Windows PC on the same network is unaffected and doesn't get WAN dropouts.