cache lookup failure

Version : Postgresql 6.4

Situation :

whenever I try to update or delete a row int the users table I get the error message

info fmgr : cache lookup failure.

update users set username = 'new' where username = 'old';
or
delete from users where username = 'old';

I can do selects with no problems at all.

TABLES:
create table users(username varchar(20) primary key,
last_name varchar(50),
first_name varchar(50),
password varchar(20),
email_on int4,
fs_on int4,
web_access_on int4,
comments text,
email_alias varchar(50),
email_forward varchar(50),
qouta int4,
date_created datetime);

create table user_groups(username varchar(20),
group_name varchar(20));

TRIGGERS:
CREATE TRIGGER users_for_key
BEFORE DELETE OR UPDATE ON users
FOR EACH ROW
EXECUTE PROCEDURE check_foreign_key(1, 'cascade', 'username', 'user_groups', 'username');

CREATE TRIGGER users_user_groups_pri_key
BEFORE INSERT OR UPDATE ON user_groups
FOR EACH ROW
EXECUTE PROCEDURE check_primary_key('username', 'users', 'username');

I am using the supplied refint.so stored procedure module.

After I rebuild the whole database this problem seems to disappear, but, will eventually reappear after a few hours. I will then have to rebuild again.

Rebuilding the database is not an acceptable work around nor fix.

Could someone please lead me in the right direction ?

Thanks,
Brian Bradley