System lockup

Two systems both Red Hat running Postgres 7.3.2
Postgres (installed from tar) data is 1.8G total space (half of which is
static data touched once a night when I vacuum), there are web pages
(Apache 2.0.44, PHP 4.3.2), and scripts (also PHP 4.3.2) intereacting
with both boxes continuously. The only postgres.conf settings that are
moved from default are:
tcpip_socket = true
max_connections = 75
shared_buffers = 150

log_connections = true
log_pid = false
log_statement = true
log_duration = true
log_timestamp = true

stats_command_string = true
stats_row_level = true
stats_block_level = true

System 1:
Red Hat Linux release 8.0 (Psyche) [default server config without X]
kernel version 2.4.18
total system RAM 1.5G
only two services running on the system Postgres and sshd
OpenSSL 0.9.6b [engine] 9 Jul 2001
output from vmstat after 20 hours of uptime (look at the memory):
procs memory swap io system

r b w swpd free buff cache si so bi bo in cs us
0 0 0 0 46080 60348 1382608 0 0 10 6 265 11 0

System 2:
Red Hat Linux release 7.1 (Seawolf) [default server config without X]
kernel version 2.4.2
total system RAM 1G
only three services running on the system Postgres, sshd, and sendmail
(hadn't turned it off yet)
OpenSSL 0.9.6b [engine] 9 Jul 2001
output from vmstat after 20 hours of uptime (look at the memory):
procs memory swap io system
r b w swpd free buff cache si so bi bo in cs us
0 0 0 0 5988 25544 915880 0 0 3 4 58 34 2

I've been running postgres for years on Red Hat systems without a hitch,
but I can't for the life of me figure this one out. So, any help would
be appreciated.