vacuumdb: PANIC: corrupted item pointer

Hello,

System: Red Hat Linux 4 64bits running postgres-7.4.16 (production)

Initial problem:

# pg_dump -O dbname -Ft -f /tmp/database.tar
pg_dump: query to get table columns failed: ERROR:  invalid memory
alloc request size 9000688640

After some research, it seems to be related to a corruption of the
database. Running a vacum crashes the db:

-bash-3.00$ vacuumdb -z -a
vacuumdb: vacuuming database "template1"
vacuumdb: vacuuming of database "template1" failed: PANIC:  corrupted
item pointer: offset = 3336, size = 20
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
-bash-3.00$

Can someone help me to track down the problem ?

Can I recover the datas (last backup failed) ?

Best regards,

Alain.