The database slows down after a few weeks

Hello

We have a problem with our database. It is accessed by
Java using Hibernate. When the program has been
running for a couple of weeks some transactions are
delayed and are executed up to an hour later than then
they are first called.

If the computer is restarted it will take about a week
before the problems appear again.

If the following sequence of commands is executed
(reinserting the data) then it will take a couple of
weeks before the problem appears again.

pg_dump > file.sql
dropdb
createdb
psql < file.sql

Is this behaviour something seen before and in that
case: how to get around it? I also need to know why it
is happening so that when I have a solution I can
guarantee that the problem will not appear again.

The system with the problem is unfortunately placed
abroad with very limited remote access. Therefore "Try
this, and try that"-solutions are not possible in this
case. I have a hard time reproducing the error on my
local system which makes the problem so much harder.

If no cause can be found: Are there any other useful
tips?

Versions running:
Linux Fedora Core 3
psql (PostgreSQL) 7.4.8
jdk1.5.0_02

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