Problem with SELECT on large negative INT4

I have a DB with is updated using MS Access. Primary keys are Int4 with default random values ("NuméroAuto" + "Aléatoire" in Access).
The DB is migrated as-is in Postgres, with tbl_prod.cle_prod field containing values from -2057496808 to 2139583719.
When I SELECT in the table, using the INT4 cle_prod value, PG doesn't find the tuple. When I SELECT using the VARCHAR(10) ref_prod value, PG finds the tuple, and show the right value for the cle_prod filed : the same as the one I SELECTed for...

This sounds like the long negative integer values given in PSQL is not converted correctly while executing.
Using a long positive integer value, all works like a charm...

Below is the queries type sto sho what append.
I'm using Postgres 6.5.2 from the RPMs.

Nicolas Huillard

gest=> select cle_prod, ref_prod from tbl_prod where ref_prod='4866VC';
cle_prod|ref_prod
-----------+--------
-2057496808|4866VC
(1 row)

gest=> select cle_prod, ref_prod from tbl_prod where cle_prod = -2057496808;
cle_prod|ref_prod
--------+--------
(0 rows)

gest=> \d tbl_prod
Table = tbl_prod
+----------------------------------+----------------------------------+-------+
| Field | Type | Length|
+----------------------------------+----------------------------------+-------+
| cle_prod | int4 not null default nextval('" | 4 |
| ref_prod | varchar() not null | 10 |
...
+----------------------------------+----------------------------------+-------+
Index: tbl_prod_pkey

gest=> select version();
version
--------------------------------------------------------------
PostgreSQL 6.5.2 on i486-pc-linux-gnu, compiled by gcc 2.7.2.3
(1 row)

gest=>