Cache lookup failure for index during pg_dump

I have a job that summarizes some data from a table, truncates the table, drops the indexes, writes the summarized data
backinto the table, then recreates the indexes.  The operations above are not in a single transaction, but separate
statementsexecuted by a script.   Easy, runs great, has for years.   

Recently the job takes a little longer to run and is still going when the database dump starts. That's when I started
gettingthis: 

ERROR:  cache lookup failed for index 70424
STATEMENT:  SELECT t.tableoid, t.oid, t.relname AS indexname, pg_catalog.pg_get_indexdef(i.indexrelid) AS indexdef,
t.relnattsAS indnkeys, i.indkey, i.indisclustered, c.contype, c.conname, c.tableoid AS contableoid, c.oid AS conoid,
(SELECTspcname FROM pg_catalog.pg_tablespace s WHERE s.oid = t.reltablespace) AS tablespace,
array_to_string(t.reloptions,', ') AS options FROM pg_catalog.pg_index i JOIN pg_catalog.pg_class t ON (t.oid =
i.indexrelid)LEFT JOIN pg_catalog.pg_depend d ON (d.classid = t.tableoid AND d.objid = t.oid AND d.deptype = 'i') LEFT
JOINpg_catalog.pg_constraint c ON (d.refclassid = c.tableoid AND d.refobjid = c.oid) WHERE i.indrelid =
'56448'::pg_catalog.oidORDER BY indexname 

The oid value changes daily, of course.  pg_dump then disconnects and stops.  I checked the source code and the query
isdefinitely coming from pg_dump.  When I run the dump a few minutes later it works beautifully, so I don't think the
systemcatalogs are corrupt. 

My questions are: can making DDL changes during a dump cause this error?  Are the queries used by pg_dump
transactionallyconsistent, i.e. do they run in a transaction and get a single view of the database system catalogs?
Otherthan finer coordination of jobs, how can this situation be avoided? 

I'm running PG 8.4.1 on linux.

Thanks in advance for your responses.

Bob Lunney