Index usage with slow query

Hi all,

Looking for some advice regarding a slow query I have and indexing.

I'm using postgresql 9.1 and this is my table that has around 6800000 rows:

CREATE TABLE mytable
(
class character varying,
floor character varying,
source_id integer,
the_geom geometry
)
WITH (
OIDS=TRUE
);

INDEX idx_source_id
USING btree
(source_id);

INDEX idx_the_geom_gist
USING gist
(the_geom);

This table is constantly hit with the below query (not always the same
values in the where). The only difference between queries are the values in
the where clause:

SELECT the_geom,oid from mytable
WHERE
the_geom && ST_GeomFromText('POLYGON((529342.334095833
180696.221733333,529342.334095833 181533.44595,530964.336820833
181533.44595,530964.336820833 180696.221733333,529342.334095833
180696.221733333))',find_srid('','mytable','the_geom'))
AND
(floor = 'gf' AND source_id = '689' AND class = 'General')

As the table has increased in size, this query has become slower, so I made
this index:

INDEX idx_floor_sourceid_class
USING btree
(floor, source_id, class);

When I run an EXPLAIN and EXPLAIN ANALYZE the query isn't using this new
index.

Sometimes it uses just idx_the_geom_gist

other times it uses idx_the_geom_gist and idx_source_id

I don't understand why it's inconsistent in its use of indexes when the
query is always the same structure, just different where clause values, and
I don't understand why it's not using the new index either.

Would love some help with this. I'm not sure where I'm going wrong.

Thanks in advance.