Clock delays in FreeBSD guest VM on VirtualBox

[Hashimoto]

Hello.

I am running several FreeBSD(8.0-BETA3) guest VMs
on VirtualBox on OpenSolaris.
On all hosts, I am configuring & running ntpd.

However, only one host (naming HostX) can sync the clock.
All hosts except for HostX cannot sync the clock.
(The time delays about 10 minutes in an hour.)
What's the cause of this problem?
Regards.

/etc/rc.conf
ntpd_enable="YES"
ntpd_sync_on_start="YES"

/etc/ntp.conf (default config file)
server 0.freebsd.pool.ntp.org iburst maxpoll 9
server 1.freebsd.pool.ntp.org iburst maxpoll 9
server 2.freebsd.pool.ntp.org iburst maxpoll 9

-- 
Kouki Hashimoto
hsmtkk at gmail.com