optimize self-join query

I have a table that stores a very large starting number called epc_start_numeric and a quantity. I've apparently built the most inefficient query possible for doing the job I need: find out if any records overlap. Imagine the epc_start_numeric + quantity representing a block of numbers. I need to find out if any of these blocks overlap.

Here is the table:

CREATE TABLE ftp_epc_audit
(
record_id serial NOT NULL,
sent_filename text NOT NULL,
pcid text NOT NULL,
tsid text NOT NULL,
user_sec_id text NOT NULL,
format_set text NOT NULL,
format text NOT NULL,
epc_start text NOT NULL,
quantity integer,
epc_decimal_start numeric(29)
)
WITH OIDS;
ALTER TABLE ftp_epc_audit OWNER TO postgres;

And the query is currently this:

SELECT '', 0, a.*, '', 0, b.* FROM ftp_epc_audit_staging a,
ftp_epc_audit_staging b
WHERE a.sent_filename <> b.sent_filename
AND a.quantity > 0
AND b.quantity > 0
AND a.locked = 1
AND b.locked = 1
AND(( a.epc_decimal_start BETWEEN b.epc_decimal_start AND b.epc_decimal_start + b.quantity - 1 )
OR ( a.epc_decimal_start + a.quantity - 1 BETWEEN b.epc_decimal_start AND b.quantity - 1 )
OR ( a.epc_decimal_start + a.quantity - 1 < b.epc_decimal_start AND a.epc_decimal_start + a.quantity -1 > b.epc_decimal_start + b.quantity - 1 ))

The column sent_filename is the unique value used so that a record does not find itself. I've got an index on sent_filename, quantity, locked and epc_decimal_start.

The query runs fine with a very small number of records. However, I need to process 60,000 records and this is taking hours. There must be a fundemental flaw in either the query or my overall design because it doesn't seem like a challenging task.

I've also tried a variant of this query which also takes several hours to run through 60,000 records.

SELECT * FROM ftp_epc_audit_staging a
WHERE a.quantity > 0
AND a.locked = 1
AND EXISTS ( SELECT TRUE FROM ftp_epc_audit_staging b WHERE b.locked = 1
AND b.quantity > 0 AND a.sent_filename <> b.sent_filename
AND(( a.epc_decimal_start BETWEEN b.epc_decimal_start AND b.epc_decimal_start + b.quantity - 1 )
OR ( a.epc_decimal_start + a.quantity - 1 BETWEEN b.epc_decimal_start AND b.quantity - 1 )
OR ( a.epc_decimal_start + a.quantity - 1 < b.epc_decimal_start AND a.epc_decimal_start + a.quantity -1 > b.epc_decimal_start + b.quantity - 1 )))

I would really appreciate any thoughts on this.