Autovacuum seems to block database: WARNING worker took too long to start

Hi all,

I'm having a strange problem with postgres & autovacuum
Everything is working fine until I start getting the following errors ...
and postgres stops working shortly after (it stops accepting connections)

2010-11-13 12:34:08.599 CET|1|||7104||4cde77b0.1bc0|2010-11-13 12:34:08
CET|1/44303|0|| LOG: automatic vacuum of table
"mrs.pg_catalog.pg_statistic": index scans: 1
pages: 0 removed, 189 remain
tuples: 132 removed, 4587 remain
system usage: CPU 0.00s/0.00u sec elapsed 0.04 sec
2010-11-13 13:24:40.998 CET|2|||3300||4cdc2ae6.ce4|2010-11-11 18:41:58
CET||0|| WARNING: worker took too long to start; cancelled
2010-11-13 13:25:41.126 CET|3|||3300||4cdc2ae6.ce4|2010-11-11 18:41:58
CET||0|| WARNING: worker took too long to start; cancelled
2010-11-13 13:26:41.254 CET|4|||3300||4cdc2ae6.ce4|2010-11-11 18:41:58
CET||0|| WARNING: worker took too long to start; cancelled
.........
.........
infinite errors like this ...
.........
.........

This situation obviously creates serious problems for us!! He have to
restart the database to make it work again.

Some info of our system

OS: Centos 5.5
Kernel: 2.6.18-194.26.1.el5
Postgres version: 8.4.5 (installation out-of-the-box using yum)
Server memory: 28GB

Autovacuum configuration
autovacuum = on
vacuum_cost_delay = 10
log_autovacuum_min_duration = 0
autovacuum_naptime = 60
autovacuum_vacuum_threshold = 50
autovacuum_vacuum_cost_delay = 20ms

Shared memory value
shared_buffers = 917504

I've read the problem seems to be autovacuum using all resources and leaving
the system unasable (not allowing to accept more connections), but I've not
read the way to fix it.
Does upgrading to postgres 9.0 help?

Any help would be appreciated!

Thanks

Pablo