changed ip-adress, DNS lookups don't work anymore

[n dhert]

FreeBSD 8.2. system.
Gets is TCP/IP parameters (and DNS name-servers IPs) from a DHCP server,
with a fixed IP address
(the system always gets the same IP, based on its MAC address as specified
in the DHCP config file)

Now I wanted the system to have a different IP address.
Changed the DHCP server config accordingly.
Reboot. OK, from  $ ifconfig -a I can see it received the new IP.

But DNS lookups don't work any longer ..
$ host xxx.yyy.zzz.com
;; connection timed out; no servers could be reached

The system is behind a firewall, but there are NO errors logged relating
to the (new) IP address.
Other FreeBSD-8.2 systems using the same DHCP server, configured in exactly
the same way, work perfectly well.

I can SSH to the sytem, but it takes 20 or 30 seconds before the Password:
prompt appears (normally should
be immediate)
Once in the system, starting my alpine mail-client, it takes a minute or so
to display the messages (normally this should be immediate)
Also at boot of the system there is wait for a 2,5 minutes somewhere in the
series of Starting <deamon>.
Probably these three phenomena have the same cause: DNS lookups don't work

any idea what can be wrong ??
I've looked and compared with other systems, can't find it ...