I have a strange problem with one of our servers (PostgreSQl 9.2.8). Every monday the autovacuum workers seems to stop working (the workers decrease in number) this causes that 5 tables that are constantly updated and deleted to be really slow.
Best guess is you have a weekly query being run that requires significant locking to execute and is thus preventing vacuum from doing its thing.
pg_stat_activity and pg_locks should be consulted during those times to gain a better feel for everything that is running during those troughs.