Invalid page feader on Solaris 10

Hi!

We have a Sun Fire V480 server 4GB of memory with Solaris 10 operation system and PostgreSQL 8.1.3 (from Sun).
The dump of my database is about 2 GB if we are lucky.
If not, we have one of error messages:

- pg_dump: ERROR: invalid memory alloc request size 2623204397

- pg_dump: ERROR: could not access status of transaction 2423378468
DETAIL: could not open file "pg_clog/0907": No such file or directory

- pg_dump: ERROR: invalid page header in block 16147 of relation "jn_szll"

What is the problem?

Some line of postgresql.conf: (the others are default)

shared_buffers = 50000 #1000 # min 16 or max_connections*2, 8KB each
#temp_buffers = 1000 # min 100, 8KB each
#max_prepared_transactions = 5 # can be 0 or more
# note: increasing max_prepared_transactions costs ~600 bytes of shared memory
# per transaction slot, plus lock space (see max_locks_per_transaction).
work_mem = 81920 #1024 # min 64, size in KB
maintenance_work_mem = 16384 # min 1024, size in KB
#max_stack_depth = 2048 # min 100, size in KB

# - Free Space Map -

max_fsm_pages = 80000 #20000 # min max_fsm_relations*16, 6 bytes each
max_fsm_relations = 4000 #1000 # min 100, ~70 bytes each

autovacuum = on # enable autovacuum subprocess?
autovacuum_naptime = 600 # time between autovacuum runs, in secs

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