Interesting thing is that even with AOLserver down, going to psql and trying to delete a row from a table would cause the backend to die. So it had nothing to do with AOLserver.
So after wandering for a while, I dropdb'd and reloaded my database from scratch from the openacs datamodel... Voila ! No more instabilities... PostgreSQL has been working like a charm.
Could this mean that there's a problem with pg_dump ?