I experienced the same behaviour here, although my ISP always supplies
primary and secondary nameserver and I therefore did never change the
respective settings in config.xml.
However, this is one of the ISP's that routinely disconnect the PPPoE line
every 24h. With 1.3b7 (and prior versions) this was no problem as those
re-connect automatically. Both 1.3b8 and 1.3b9 do not re-connect
automatically on my Alix 2c3.
For me going back to 1.3b7 solved the problem for the time being.