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