increased load on server

Hi,

I'm using postgreSQL 8.3 on Ubuntu 8.04 with ltree.
I tried to tune posgresql.conf to use more memory.

In two years on production everything was fine, but today something
goes wrong: postgresql processes eat my server ressources, my load,
which was between 0 and 2.5 on my 4 cpus server, grows to 20.

When the trouble appends there was a lot of waiting queries on my ltree table.
I tried to restart postgresql server and to reboot the server, but 5
minutes later, the trouble was here again.

I did a backup/restore of my database (pg_dump -Fc / pg_restore), and
it was fine for 2 hours but after the touble reappears.

I notice that the "base" directory grows a lot when the trouble
begins. I made a 'vacuum', 'vacuum analyze', 'vacuum full' but the
disk usage isn't lower.

Please have you a clue to help me ? This is the week-end and I'm
hoping to fix this before monday.

Thanks.