Portsnap causes system to reboot

[Walter Venable]

Whenever I run portsnap fetch update (edit: it also happens for a
simple portsnap fetch), my system reboots unexpectedly. Here's the
output:
# portsnap fetch update
Looking up portsnap.FreeBSD.org mirrors... 3 mirrors found.
Fetching snapshot tag from portsnap2.FreeBSD.org... done.
Fetching snapshot metadata... done.
Updating from Wed Sep 24 00:04:04 EEST 2008 to Thu Oct  9 10:28:42 EEST 2008.
Fetching 3 metadata patches.. done.
Applying metadata patches... done.
Fetching 3 metadata files... done.
Fetching 602 patches.....10....20....30....40....50....60....70....80....90....100....110....120....130....140....150....160....170....180....190....200....210....220....230....240....250....260....270....280....290....300....310....320....330....340....350....360....370....380....390....400....410....420....430....440....450....460....470....480....490....500....510....520....530....540....550....560....570....580....590....600.
done.
Applying patches... Read from remote host X: Connection reset by peer
Connection to X closed.

And then I can log-in again a few minutes later, and the uptime has
gone down to a few seconds, so I know it rebooted. Any ideas why this
is happening?

Some background info:
$ uname -mrs
FreeBSD 7.0-RELEASE-p5 i386

And:
$ cat /etc/make.conf
# added by use.perl 2008-07-16 15:32:01
PERL_VER=5.8.8
PERL_VERSION=5.8.8

CFLAGS=-O2 -pipe
COPTFLAGS=-O2 -pipe
CPUTYPE=athlon-xp

NO_PROFILE=true

Since this started happening, I have still successfully updated ports
by csup'ing the ports tree. I can also still rebuild the world and
kernel without issue.  This is a remote box, and I don't use X with
it.