cache lookup of relation 165058647 failed

I have a problem with postgresql tables. periodicaly, I would say
frequently about 5-10 time per hour i have such errors in my server log
file:

2004-04-14 12:23:32 [73692] ERROR: cache lookup of relation 149064743
failed
2004-04-14 12:23:32 [73692] ERROR: Relation "tmp_table1" does not exist
2004-04-14 12:23:32 [73692] ERROR: Relation "tmp_table1" does not exist

So turn on debugging options and have that's what i got:

2004-04-14 12:24:54 [74021] LOG: connection received: host=[local]
2004-04-14 12:24:54 [74021] LOG: connection authorized: user=ejob_guest
database=ejob
2004-04-14 12:24:54 [74021] LOG: query: begin; select
getdatabaseencoding(); commit
2004-04-14 12:24:54 [74021] LOG: duration: 0.000987 sec
2004-04-14 12:24:54 [74021] LOG: query: SELECT d.config_id, d.text1 as
text, d.field_name,
2004-04-14 12:24:54 [74021] LOG: duration: 0.005295 sec
2004-04-14 12:24:54 [74021] LOG: query: CREATE TEMP TABLE tmp_table1 AS
SELECT advert.id, (SELECT employer.fullname FROM employer WHERE
advert.empid=employer.id) as emp_name, (CASE WHEN advert.status1 = 'A'
THEN advert.postname1 ELSE advert.postname2 END) as postname, (SELECT
cd.name FROM catalog_names cd WHERE advert.industry=cd.catalog_id AND
cd.language_id=1) AS f1002, (CASE WHEN status1='A' THEN
advert.obligations1 ELSE advert.obligations2 END) as obligations1 FROM
good_adv as advert
2004-04-14 12:24:54 [74021] ERROR: cache lookup of relation 165058647
failed
2004-04-14 12:24:54 [74021] LOG: query: SELECT * FROM tmp_table1
2004-04-14 12:24:54 [74021] ERROR: Relation "tmp_table1" does not exist
2004-04-14 12:24:54 [74021] LOG: query: SELECT * FROM tmp_table1 ORDER
BY id desc LIMIT 100
2004-04-14 12:24:54 [74021] ERROR: Relation "tmp_table1" does not exist

Does anybody have any idea about what can cause such kind of problems and
how to overcome it ?

Thanks a lot.