make buildworld powers down system

[doug]

Three times in a row. It seems to be repeatable. At the end what should my next 
debugging step be?

I ran the buildworld using script, the last lines in the make:

cc -O2 -pipe  -D_XOPEN_SOURCE_EXTENDED -DENABLE_WIDEC -I. 
-I/usr/obj/usr/src/lib/ncurses/ncursesw/../ncursesw 
-I/usr/src/lib/ncurses/ncursesw/../ncursesw 
-I/usr/src/lib/ncurses/ncursesw/../ncurses 
-I/usr/src/lib/ncurses/ncursesw/../../../contrib/ncurses/include 
-I/usr/src/lib/ncurses/ncursesw/../../../contrib/ncurses/ncurses -Wall -DNDEBUG 
-DHAVE_CONFIG_H -DFREEBSD_NATIVE -DTERMIOS -std=gnu99 -fstack-protector  -c 
/usr/src/lib/ncurses/ncursesw/../../../contrib/ncurses/ncurses/base/lib_beep.c
cc -O2 -pipe  -D_XOPEN_SOURCE_EXTENDED -DENABLE_WIDEC -I. 
-I/usr/obj/usr/src/lib/ncurses/ncursesw/../ncursesw 
-I/usr/src/lib/ncurses/ncursesw/../ncursesw 
-I/usr/src/lib/ncurses/ncursesw/../ncurses 
-I/usr/src/lib/ncurses/ncursesw/../../../contrib/ncurses/include 
-I/usr/src/lib/ncurses/ncursesw/../../../contrib/ncurses/ncurses -Wall -DNDEBUG 
-DHAVE_CONFIG_H -DFREEBSD_NATIVE -DTERMIOS -std=gnu99 -fstack-protector  -c 
/usr/src/lib/ncurses/ncursesw/../../../contrib/ncurses/ncurses/base/lib_bkgd.c
cc -O2 -pipe  -D_XOPEN_SOURCE_EXTENDED -DENABLE_WIDEC -I. 
-I/usr/obj/usr/src/lib/ncurses/ncursesw/../ncursesw 
-I/usr/src/lib/ncurses/ncursesw/../ncursesw 
-I/usr/src/lib/ncurses/ncursesw/../ncurses 
-I/usr/src/lib/ncurses/ncursesw/../../../contrib/ncurses/include 
-I/usr/src/lib/ncurses/ncursesw/../../../contrib/ncurses/ncurses -Wall -DNDEBUG 
-DHAVE_CONFIG_H -DFREEBSD_NATIVE -DTERMIOS -std=gnu99 -fstack-protector  -c 
/usr/src/lib/ncurses/ncursesw/../../../contrib/ncurses/ncurses/base/lib_box.c
cc -O2 -pipe  -D_XOPEN_SOURCE_EXTENDED -DENABLE_WIDEC -I. 
-I/usr/obj/usr/src/lib/ncurses/ncursesw/../ncursesw 
-I/usr/src/lib/ncurses/ncursesw/../ncursesw 
-I/usr/src/lib/ncurses/ncursesw/../ncurses 
-I/usr/src/lib/ncurses/ncursesw/../../../contrib/ncurses/include 
-I/usr/src/lib/ncurses/ncursesw/../../../contrib/ncurses/ncurses -Wall 
-DNDE^@^@^@^@^@^@^@

this after about 30 minutes of running. There are no errors in the logs

I used 'csup -g -L 2 8.2_supfile' to build/update /usr/src

8.2_supfile
  *default host=cvsup10.FreeBSD.org
  *default base=/var/db
  *default prefix=/usr
  *default release=cvs tag=RELENG_8_2
  *default delete use-rel-suffix
  *default compress
  src-all

I did a find /usr/src -type f -mtime -180

  /usr/src/UPDATING
  /usr/src/sys/compat/linux/linux_socket.c
  /usr/src/sys/conf/newvers.sh
  /usr/src/sys/kern/uipc_usrreq.c
  /usr/src/usr.bin/compress/zopen.c
  /usr/src/usr.bin/gzip/zuncompress.c

I compared this with an another 8.2_rel system built with freebsd-update. The 
same command added only /usr/src/usr.sbin/mountd/mountd.c. This file differed 
only in the fbsd id header.

Next I ran fsck on /usr and /var from single-user mode, finding no errors.

I have no evidence of disk errors and the system runs windows 7 and FreeBSD 
wihtout any strange happenings.

What should my next step be?