> I powered the router down again, plugged its WAN port into one of the
> LAN ports of the ISP-supplied router, and brought it back up.
I you sure you plugged your ISP-router into the WAN port of your
(Buffalo) router and not into one of the LAN ports?
The behavior you describe would be easy to explain if it was plugged
into a LAN port (or if the WAN port was somehow bridged with the LAN
ports) since in that case you'd have basically a single network with
packets forwarded between the two routers, and two DHCP servers, making
it quite possible that a DHCP request received on your router ends up
being answered by the ISP router instead (since the request is
broadcasted to all connected machines).
Stefan
