canceling autovacuum task error

Hi All

I am using a cluster setup having postgres-8.4.0 and slon 2.0.4 is being
used for replication . It happened that the autovacuum was not running
successfully on one of the nodes in cluster and was giving error :

2011-05-13 23:07:42 CDTERROR: canceling autovacuum task
2011-05-13 23:07:42 CDTCONTEXT: automatic vacuum of table
"abc.abc.sometablename"
2011-05-13 23:07:42 CDTERROR: could not open relation with OID 141231 at
character 87

sometimes it was giving a different error as below :

2011-05-13 04:45:05 CDTERROR: canceling autovacuum task
2011-05-13 04:45:05 CDTCONTEXT: automatic analyze of table
"abc.abc.sometablename"
2011-05-13 04:45:05 CDTLOG: could not receive data from client: Connection
reset by peer
2011-05-13 04:45:05 CDTLOG: unexpected EOF on client connection
2011-05-13 04:45:05 CDTERROR: duplicate key value violates unique
constraint "sl_nodelock-pkey"
2011-05-13 04:45:05 CDTSTATEMENT: select "_schemaname".cleanupNodelock();
insert into "_mswcluster".sl_nodelock values ( 2, 0,
"pg_catalog".pg_backend_pid());

Can see the below log also in postgres logs :

"checkpoints are occurring too frequently (19 seconds apart)"
I am not sure when these all errors started coming . Just noticed these
when database size grew huge and it became slow.

Can anybody shed some light on it if these errors are related or what could
be the reason for these errors .

Thanks..
Tamanna