networking "restart" drops eth0 :(

[David]

I'm just building a brand new Ubuntu server box. Unfortunately I put in
the wrong address for the nameserver so I changed it manually (edited
/etc/resolv.conf and /etc/network/interfaces) and now if I restart
networking I lose the eth0 connection completely! ie, ifconfig only shows
loopback.

If I completely reboot the machine, eth0 comes back and works fine until
the next time I restart networking :(

Can anyone suggest what I'm doing wrong?

David.