RC2 intermittent errors

Hi all,
I'm testing now RC2 against our application and I'm experiencing
intermittent errors. I isolated this test:

CREATE TABLE users (
id_login SERIAL PRIMARY KEY,
login TEXT
);

CREATE OR REPLACE FUNCTION sp_id_user ( TEXT )
RETURNS INTEGER AS $$
DECLARE
a_login ALIAS FOR $1;
my_id INTEGER;
BEGIN
SELECT id_login INTO my_id
FROM users
WHERE login = a_login;

RETURN COALESCE(my_id, -1 );

END;
$$ LANGUAGE 'plpgsql'
STABLE;

CREATE OR REPLACE FUNCTION sp_test ( TEXT )
RETURNS INTEGER AS $$
DECLARE
a_login ALIAS FOR $1;
my_id INTEGER;
BEGIN
my_id = sp_id_user( a_login );
RAISE NOTICE 'ID> %', my_id;

insert into users (login) values ( a_login );

my_id = sp_id_user( a_login );
RAISE NOTICE 'ID> %', my_id;

RETURN 0;
END;
$$ LANGUAGE 'plpgsql';

select sp_test('test1');
select sp_test('test2');

The call of the two above functions shall show:

ID> -1
ID> 1

ID> -1
ID> 2

instead I have:

test=# select sp_test('test1');
NOTICE: ID> -1
NOTICE: ID> 1
sp_test
---------
0
(1 row)

test=# select sp_test('test2');
NOTICE: ID> -1
NOTICE: ID> -1
sp_test
---------
0
(1 row)

some times I get:

test=# select sp_test('test1');
NOTICE: ID> -1
NOTICE: ID> -1
sp_test
---------
0
(1 row)

test=# select sp_test('test2');
NOTICE: ID> -1
NOTICE: ID> -1
sp_test
---------
0
(1 row)

Regards
Gaetano Mendola