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.