missing chunk number 0 for toast value

This is for PostgreSQL 7.4.6 on hppa2.0w-hp-hpux11.00, compiled
by GCC 3.2.2.  I'm seeing the following 2 errors:

ERROR:  missing chunk number 0 for toast value 19319495
ERROR:  unexpected chunk number 4 (expected 0) for toast value
19319495

It is always the same toast value.  The table has 1.8M rows.
I've identified the column and the row:

$ psql -c "\d audit"
               Table "public.audit"
 Column |            Type             | Modifiers
--------+-----------------------------+-----------
 value  | text                        |
 cdate  | timestamp without time zone |
 key    | text                        |
Indexes:
    "idx_audit_cdate" btree (cdate)
    "idx_audit_key" btree ("key")

$ psql -c "select key from audit where key = '791015.o9'"
    key
-----------
 791015.o9
(1 row)

$ psql -c "select cdate from audit where key = '791015.o9'"
        cdate
---------------------
 2005-04-06 00:00:00
(1 row)

$ psql -c "select value from audit where key = '791015.o9'"
ERROR:  missing chunk number 0 for toast value 19319495

It causes all dumps and any access (via SELECT, UPDATE, and
COPY/pg_dump) to audit.value to fail.  I've tried to look at the
underlying toast table per some old posts, but no joy.

As usual, this is on a system with a downtime-sensitive customer.

Any ideas?

Ed