Spontaneous PostgreSQL Server Reboot?

Help!

I'm having a severe problem with my production PostgreSQL servers, and
I'm running out of ideas as to what could be the problem.

On three different machines running the same PostgreSQL version (7.3.5)
on Linux and almost identical databases, I have been plagued by
occasional, unexplainable (to me) reboots of the computer.

On the first two machines it was clearly connected to running a very
complex query (multi-hour processing time);  on the third, I could find
no such correlation.  Please note that only one machine was Production
at any one time;  the problem never occurred with the Development
servers (equivalent configuration).  This in and of itself may hint to a
threading issue (the DB is serving web servers, plus the occasional
maintenance cron job - no cron job running when the last reboot occurred).

When the last reboot occurred (this past Friday evening), I scoured all
logs for any information;  the only log that shows any activity in
within 5 minutes of the restart is $PG_DATA/serverlog:

2004-03-26 21:46:59 [10534]  LOG:  query: SELECT  v.* FROM [...]
2004-03-26 21:46:59 [10534]  LOG:  duration: 0.049330 sec

The machine restarted around 21:50 (based on syslog):

Mar 26 21:50:33 prod-db-2-1 syslogd 1.4.1#10: restart.

I especially found no panics, errors, warnings, ...  The machine is used
only as a DB server, nothing else running (not even a mail server).  I'm
running out of things to look for!

I'm running PostgreSQL 7.3.5 on Linux (pentium machines), configured as
follows:
1. Red Hat 7.3 with 120 GB HDD and 1 GB RAM;
2. Red Hat 7.3 with 80 GB HDD and 512 MB RAM;
3. Debian with a 2.2.20 kernel, 5.5 GB HDD and 384 MB RAM.  This is the
latest one in use.

Note that even the 5.5 GB machine has over 1 GB spare on the
/usr/local/pgsql/ partition.

Thanks for any and all help!

        Andrew Biagioni