query has huge variance in execution times

postgres 8.3.5 on RHEL4 update 6

This query starts executing at 18:41:

cemdb=> select query_start,current_query from pg_stat_activity where
procpid=10022; query_start |

current_query
-------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
2010-03-30 18:41:11.685261-07 | select b.ts_id from
ts_stats_tranunit_user_daily b, ts_stats_tranunit_user_interval c where
b.ts_transet_incarnation_id = c.ts_transet_incarnation_id and
b.ts_tranunit_id = c.ts_tranunit_id and b.ts_user_incarnation_id =
c.ts_user_incarnation_id and c.ts_interval_start_time >= $1 and
c.ts_interval_start_time < $2 and b.ts_interval_start_time >= $3 and
b.ts_interval_start_time < $4
(1 row)

about 5 mins later, I, suspecting problems, do (the values are the same
as for $1 et al above; EXPLAIN was done on purpose to keep stats
[hopefully] the same as when pid 10022 started; there are 80,000 rows in
each of the 2 tables at the time of this EXPLAIN and when 10022 started):

cemdb=> explain select b.ts_id from ts_stats_tranunit_user_daily b,
ts_stats_tranunit_user_interval c where b.ts_transet_incarnation_id =
c.ts_transet_incarnation_id and b.ts_tranunit_id = c.ts_tranunit_id and
b.ts_user_incarnation_id = c.ts_user_incarnation_id and
c.ts_interval_start_time >= '2010-3-29 01:00' and
c.ts_interval_start_time < '2010-3-29 02:00' and
b.ts_interval_start_time >= '2010-3-29' and b.ts_interval_start_time <
'2010-3-30';

QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Merge Join (cost=33574.89..34369.38 rows=25207 width=8)
Merge Cond: ((b.ts_transet_incarnation_id =
c.ts_transet_incarnation_id) AND (b.ts_tranunit_id = c.ts_tranunit_id)
AND (b.ts_user_incarnation_id = c.ts_user_incarnation_id))
-> Sort (cost=13756.68..13854.96 rows=78623 width=32)
Sort Key: b.ts_transet_incarnation_id, b.ts_tranunit_id,
b.ts_user_incarnation_id
-> Index Scan using ts_stats_tranunit_user_daily_starttime on
ts_stats_tranunit_user_daily b (cost=0.00..10560.13 rows=78623 width=32)
Index Cond: ((ts_interval_start_time >= '2010-03-29
00:00:00-07'::timestamp with time zone) AND (ts_interval_start_time <
'2010-03-30 00:00:00-07'::timestamp with time zone))
-> Sort (cost=19818.21..19959.72 rows=113207 width=24)
Sort Key: c.ts_transet_incarnation_id, c.ts_tranunit_id,
c.ts_user_incarnation_id
-> Index Scan using ts_stats_tranunit_user_interval_starttime
on ts_stats_tranunit_user_interval c (cost=0.00..15066.74 rows=113207
width=24)
Index Cond: ((ts_interval_start_time >= '2010-03-29
01:00:00-07'::timestamp with time zone) AND (ts_interval_start_time <
'2010-03-29 02:00:00-07'::timestamp with time zone))
(10 rows)

cemdb=> \q

I then run the query manually:

[root(at)rdl64xeoserv01 log]# time PGPASSWORD=quality psql -U admin -d
cemdb -c "select b.ts_id from ts_stats_tranunit_user_daily b,
ts_stats_tranunit_user_interval c where b.ts_transet_incarnation_id =
c.ts_transet_incarnation_id and b.ts_tranunit_id = c.ts_tranunit_id and
b.ts_user_incarnation_id = c.ts_user_incarnation_id and
c.ts_interval_start_time >= '2010-3-29 01:00' and
c.ts_interval_start_time < '2010-3-29 02:00' and
b.ts_interval_start_time >= '2010-3-29' and b.ts_interval_start_time <
'2010-3-30'" > /tmp/select.txt 2>&1

real 0m0.813s
user 0m0.116s
sys 0m0.013s

I let process 10022 run for an hour. an strace shows lots of I/O:

[root(at)rdl64xeoserv01 log]# strace -p 10022
read(18, "\214\2\0\0\374<\200#\1\0\0\0<\0P\3\0 \4 \0\0\0\0\320\234"...,
8192) = 8192
semop(73007122, 0xbfe0fc20, 1) = 0
_llseek(18, 538451968, [538451968], SEEK_SET) = 0
read(18, "\214\2\0\0\274\347\t#\1\0\0\0<\0P\3\0 \4 \0\0\0\0\320\234"...,
8192) = 8192
_llseek(18, 535928832, [535928832], SEEK_SET) = 0
read(18, "\214\2\0\0\310\300\226\"\1\0\0\0<\0P\3\0 \4 \0\0\0\0\320"...,
8192) = 8192
_llseek(18, 532398080, [532398080], SEEK_SET) = 0

<many more similar lines>

I then kill 10022 and the application retries the same query:

[10022-cemdb-admin-2010-03-30 19:02:37.460 PDT]FATAL: terminating
connection due to administrator command
[10022-cemdb-admin-2010-03-30 19:02:37.460 PDT]STATEMENT: select
b.ts_id from ts_stats_tranunit_user_daily b,
ts_stats_tranunit_user_interval c where b.ts_transet_incarnation_id =
c.ts_transet_incarnation_id and b.ts_tranunit_id = c.ts_tranunit_id and
b.ts_user_incarnation_id = c.ts_user_incarnation_id and
c.ts_interval_start_time >= $1 and c.ts_interval_start_time < $2 and
b.ts_interval_start_time >= $3 and b.ts_interval_start_time < $4

[10820-cemdb-admin-2010-03-30 19:02:40.363 PDT]LOG: duration: 1096.598
ms execute <unnamed>: select b.ts_id from ts_stats_tranunit_user_daily
b, ts_stats_tranunit_user_interval c where b.ts_transet_incarnation_id =
c.ts_transet_incarnation_id and b.ts_tranunit_id = c.ts_tranunit_id and
b.ts_user_incarnation_id = c.ts_user_incarnation_id and
c.ts_interval_start_time >= $1 and c.ts_interval_start_time < $2 and
b.ts_interval_start_time >= $3 and b.ts_interval_start_time < $4

Ideas on why the big difference in execution times of the same query and
how to avoid same are solicited. I tend to doubt, but don't know how to
prove, that the query stats for these 2 tables were updated between the
start of pid 10022 and when I did the EXPLAIN.

Thanks,
Brian