unattended install of FreeBSD 5.2.1 does not umount disk properly

I'm trying to do an unattended install of FreeBSD 5.2.1.  If I don't
include the 'shutdown' command at the end of the install.cfg file, the
install works fine, but I have to manually reboot the machine after the
install, which is no good for an unattended install.

If I do include 'shutdown', then it appears that the disk does not unmount
properly before the reboot.  The console displays "syncing
disks, buffers remaining".  Normally I would expect a few
numbers to be printed, followed by a reboot a second later. This what
happens if the reboot is performed manually from the sysinstall menu.

But when the reboot is performed automatically, after "syncing disks"  it
says: "WARNING - WRITE_DMA interrupt was seen but timeout fired LBA" and
then prints the same number (e.g. 161) over and over again for several
lines.  After about 5 seconds the system reboots, but the new installation
has errors on the filesystem and requires me to manually run fsck. This is
no good for an unattended installation.

Is there anyway to reboot the system from the install.cfg file and have
it unmount cleanly?

-- 
Richard