Can't run vacuum....

We are getting the following error when we run
vacuum:

NOTICE: BlowawayRelationBuffers(pg_index, 2): block 2
is referenced (private 0, last 0, global 1)
pqReadData() -- backend closed the channel
unexpectedly.
This probably means the backend terminated
abnormally before or while processing the request.
We have lost the connection to the backend, so further
processing is impossible. Terminating.

It occurs randomly. The database appears to continue
to function; however, every vacuum afterwards fails.
I noticed in the mailing lists that previous users
have run into the same problem and was wondering if
any solutions have been found...?

Thanks for any information,

Marcus Mascari (mascarim(at)yahoo(dot)com)

PostgreSQL v 6.4.0
RedHat Linux 5.2
gcc version 2.7.2.3

_________________________________________________________
DO YOU YAHOO!?
Get your free @yahoo.com address at http://mail.yahoo.com