postgres getting slow under heavy load though autivacuum is enabled

Hi

I am using postgres 8.1.2 with slony 1.1.5 used for replication between two nodes. Very high number of db operations like (2.8 million inserts, 1.4 million update and 4.5 lakhs deletes.) are being done on db in one transaction and this is repeated for 5-6 times a day at an interval of let say 2 hours. This process is runnning for 5 consective days. It is obeserved that db is getting very slow with time. The number of dead tuples getting increased in pg_listener, sl_log_1 and sl_seqlog tables with time though I have autovacuum enabled and slony related tables like (sl_log_1 , sl_seqlog etc) are configured not to be processed by autovacuum . Please let me know what could be the reason of increasing dead tuples in these tables and postgres getting slow.

Thanks..
Tamanna