Any advice on debugging hanging postgresql-8.1.21 (many postmaster's)

Hello Postgres users,

I have a Linux website with phpBB serving a small Facebook game:

# uname -a
Linux XXXXX 2.6.18-194.17.1.el5 #1 SMP Wed Sep 29 12:50:31 EDT 2010
x86_64 x86_64 x86_64 GNU/Linux
# cat /etc/redhat-release
CentOS release 5.5 (Final)
# rpm -qa | grep -i postgres
postgresql-devel-8.1.21-1.el5_5.1
postgresql-8.1.21-1.el5_5.1
postgresql-server-8.1.21-1.el5_5.1
postgresql-devel-8.1.21-1.el5_5.1
postgresql-libs-8.1.21-1.el5_5.1
postgresql-libs-8.1.21-1.el5_5.1
postgresql-docs-8.1.21-1.el5_5.1

It works generally ok and with little load since June.
(My game has less than 100 users and I have AMD/Quad+4GB)

On Friday I've installed a cronjob (full source code at the bottom):

3       3       *       *       *       psql -a -f
$HOME/bin/clean-phpbb-forum.sql

and on Monday I've found out, that the site is overloaded:

top - 10:16:56 up 3 days, 23:56,  1 user,  load average: 20.55, 21.38, 22.92
Tasks: 157 total,  24 running, 133 sleeping,   0 stopped,   0 zombie
Cpu(s): 98.9%us,  1.0%sy,  0.0%ni,  0.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   4019028k total,  3073968k used,   945060k free,    50604k buffers
Swap:  2104496k total,      132k used,  2104364k free,  2316196k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
28974 postgres  16   0  122m  14m  10m R 73.3  0.4  20:56.06 postmaster
28727 postgres  16   0  121m  13m  10m R 54.7  0.3  37:58.58 postmaster
28714 postgres  16   0  122m  14m  10m R 50.4  0.4  38:38.98 postmaster
29412 postgres  16   0  121m  13m  10m R 46.5  0.4   8:03.96 postmaster
28542 postgres  16   0  122m  14m  10m R 46.2  0.4  58:49.38 postmaster
28482 postgres  16   0  122m  14m  10m R 45.8  0.4  61:53.37 postmaster
28468 postgres  16   0  122m  14m  10m R 44.2  0.4  62:46.17 postmaster
29017 postgres  16   0  122m  14m  10m R 43.9  0.4  19:17.06 postmaster
28929 postgres  15   0  122m  14m  10m R 42.2  0.4  22:01.43 postmaster
28500 postgres  16   0  122m  14m  10m R 41.3  0.4  59:40.23 postmaster
28460 postgres  16   0  122m  14m  10m R 40.6  0.4  64:17.16 postmaster
28894 postgres  16   0  122m  14m  10m R 38.6  0.4  23:35.53 postmaster
28489 postgres  16   0  122m  14m  10m R 36.0  0.4  60:32.59 postmaster
28719 postgres  15   0  121m  13m  10m R 25.2  0.3  38:10.33 postmaster
29496 postgres  16   0  121m  13m  10m R 22.9  0.4   4:20.32 postmaster
28556 postgres  15   0  122m  14m  10m R 17.7  0.4  57:32.62 postmaster
28735 postgres  15   0  122m  14m  10m R 15.7  0.4  36:09.45 postmaster
29602 postgres  15   0  119m  11m 9680 S  8.2  0.3   0:00.25 postmaster
28457 postgres  17   0  122m  14m   9m R  3.6  0.4  64:34.38 postmaster
26092 apache    15   0  238m  16m 3740 S  0.3  0.4   0:03.38 httpd
29596 afarber   15   0 12744 1116  800 R  0.3  0.0   0:00.09 top
    1 root      15   0 10352  700  592 S  0.0  0.0   0:01.69 init

I understand, that I probably supply not enough info,
but how would you approach debugging this problem?

I've run my cronjob numerous times from CLI - it works ok
and takes only few seconds. I've installed it as an every-3 mins
cronjob on my development VM - it works ok there too.

My biggest problem is, that I don't see any information
from Postgres at the production site - why did it have to start
so many postmaster's (whatever those processes do).

The only log file I've found has been /var/lib/pgsql/pgstartup.log
and there is nothing suspicious there. (Also nothing related to Postgres
in messages, mcelog (empty), audit.log, access_log, error_log).

Please give me some hints
Alex

# cat bin/clean-phpbb-forum.sql
start transaction;

delete from phpbb_users
where user_type=1 and user_inactive_reason=1 and user_lastvisit=0 and
age(to_timestamp(user_regdate))>interval '3 days';

create temp table old_topics (topic_id integer) on commit delete rows;
create temp table old_posts (post_id integer) on commit delete rows;

insert into old_topics select topic_id from phpbb_topics
where forum_id=5 and topic_poster=1 and
age(to_timestamp(topic_time))>interval '7 days';

-- select count(topic_id) as "old topics:" from old_topics;

insert into old_posts select p.post_id from phpbb_posts p, old_topics t
where p.forum_id=5 and p.poster_id=1 and p.topic_id=t.topic_id;

-- select count(post_id) as "old posts:" from old_posts;

delete from phpbb_posts where post_id in (select post_id from old_posts);
delete from phpbb_topics where topic_id in (select topic_id from old_topics);

update phpbb_config set
config_value = (select count(topic_id) from phpbb_topics)
where config_name = 'num_topics';

update phpbb_config set
config_value = (select count(post_id) from phpbb_posts)
where config_name = 'num_posts';

update phpbb_users set
user_posts = (select count(post_id) from phpbb_posts where poster_id=1)
where user_id = 1;

update phpbb_forums set
forum_posts = (select count(post_id) from phpbb_posts),
forum_topics = (select count(topic_id) from phpbb_topics),
forum_topics_real = (select count(topic_id) from phpbb_topics)
where forum_id = 5;

commit