postgres 'forgets' table definitions

Hi,

We're running pgsql 7.3.14 (yes I know it's old, an upgrade to 8.3 is planned).
Since a couple of weeks postgres loses the table definitions from apparently random tables. A 'broken' table actually still works but a \d table gives nothing. Currently The biggest problem is that these broken tables don't appear in the backups.

example:
mydb=# select * from toegevoegde_dienst_omzetting_history limit 1;
omzetting_id | toegevoegde_dienst_id | datum_aanvraag | datum_ingang | datum_uitgevoerd | product_soort_id_oud | product_soort_id_nieuw | id | door_wie | sinds
--------------+-----------------------+----------------+--------------+------------------+----------------------+------------------------+----+------------+---------------------
5 | 167817 | 2007-12-23 | 2007-12-23 | | 5816 | 5817 | 6 | webservice | 2007-12-23 19:55:36
(1 row)

mydb=# \d toegevoegde_dienst_omzetting_history
Did not find any relation named "toegevoegde_dienst_omzetting_history".
mydb=#

And this table is obviously from the pg_catalog.pg_class table as well.

Has anyone any idea what may cause this? I can't find anything in the logs regarding the missing tables. (eg. that someone messes the pg_catalog tables). I disabled the nightly vacuum process but the problem persists.

Could this problem be fixed by inserting the right records back in to in de pg_catalog tables? is there a way to dump the pg_catalog tables?

The pgserver is running on a 4 CPU dell 6850 with 16G of Ram with Fedora Core 4 on a reiserfs filesystem without any problems for years. And nothing has changed on the server side of things.

thanks,
alex.