FreeBSD cluster: ntpd does not sync time properly?

[Rob]

Hello,

I have a FreeBSD cluster, all PCs running FreeBSD-Stable (as of
one week ago). The cluster has a master, connected to the Internet,
and the master is also router, caching nameserver and timeserver
for the internal network.
The master has two internet cards, and there are 6 slaves
on the internal network.

Problem is, that as time goes on, the system time on master
and slaves deviate more and more.

Here is my configuration for the time server configuration:

on MASTER (192.168.0.100)
rc.conf:
   ntpdate_enable="YES"
   ntpdate_flags="-b time.kriss.re.kr time.nuri.net"
   xntpd_enable="YES"
ntp.conf:
   # prohibit general access to this service
   restrict default ignore
   # allow hosts on the 192.168.0.0/24 subnet to query this server
   restrict 192.168.0.0 mask 255.255.255.0 nomodify notrap
   # localhost has full access to the server
   restrict 127.0.0.1
   server time.kriss.re.kr prefer
   server time.nuri.net
   driftfile /etc/ntp.drift


on SLAVE<N> (192.168.0.<N>)
rc.conf:
   ntpdate_enable="YES"
   ntpdate_flags="-b 192.168.0.100"
   xntpd_enable="YES"
ntp.conf:
   # prohibit general access to this service
   restrict default ignore
   # localhost has full access to the server
   restrict 127.0.0.1
   server 192.168.0.100
   driftfile /etc/ntp.drift


The idea is that at boot-up, time is instantly synchronized once by
unsing "ntpdate -b"; after that ntpd will control time adjustments.

Am I making a mistake in this setup?

Thanks,
Rob.