Strange Postgresql Indexing Behavior

I'm having an interesting time trying to figure out some behavior with
postgresql indexes that I am trying to understand.

I have a table, historyticket. In that table, I have a column,
fk_opener_id, which is an integer column. I have an index (default btree
index) on the fk_opener_id column in that table.

When I select from this table with the following query:

select * from historyticket where fk_opener_id = ?

The query sometimes uses the index, and sometimes does a sequential scan.
I experimented for a little bit and found out that if the number of rows
that match the query is greater than a certain number (somewhere around
1000 rows from what I can tell) then the index is not used.

For example:

=============
testing=# explain select * from historyticket where fk_opener_id = 67;
NOTICE:  QUERY PLAN:

Seq Scan on historyticket  (cost=0.00..768.62 rows=1246 width=419)

testing=# select count(*) from historyticket where fk_opener_id = 67;
 count
-------
  1158
(1 row)
---------
testing=# explain select * from historyticket where fk_opener_id = 4;
NOTICE:  QUERY PLAN:

Index Scan using fk_opener_id on historyticket  (cost=0.00..179.47 rows=47
width=419)

testing=# select count(*) from historyticket where fk_opener_id = 4;
 count
-------
    79
(1 row)
===============

I did more queries and confirmed that when the number of rows returned is
below a certain number (I don't have enough data to determine the exact
number) the index is used, and when it is above a certain number, it is
not used.

Can anyone explain to me what is happening / why it is happening / how to
make the indexes work correctly?

Thanks.

Brian