Index not used past a certain threshold

I seem to have run into an indexing problem in postgres 7.4 on Solaris 8.
An index on a certain table is not being used if a column referenced in
the query has more than a certain number of rows for a given value.
Here's an example:

I have a table like so:

Table "public.a1"
Column | Type | Modifiers
--------+---------+-----------
aid | integer | not null
Indexes:
"a1b" hash (aid)

With an index like so:

Index "public.a1b"
Column | Type
--------+---------
aid | integer
hash, for table "public.a1"

If we group this table by "number of rows for a given aid", using this
query:
select aid, count (aid) from a1 group by aid order by count (aid) using >;
then the largest rows are:

aid | count
------+-------
4085 | 51039
5065 | 45750
5026 | 39224
9010 | 31418
527 | 30691
5014 | 29421
5010 | 24958
7 | 20723
57 | 19167
722 | 17180
[...snip...]

Now, the index is *not* being used if I query for rows
where count >= 19167.

it=> explain select aid from a1 where aid = 57;
QUERY PLAN
----------------------------------------------------------
Seq Scan on a1 (cost=0.00..17986.81 rows=15738 width=4)
Filter: (aid = 57)

And the rows with higher counts as well. But beneath that threshold, the
index is indeed being used:

it=> explain select aid from a1 where aid = 722;
QUERY PLAN
--------------------------------------------------------------------
Index Scan using a1b on a1 (cost=0.00..4796.23 rows=1337 width=4)
Index Cond: (aid = 722)

The actual table in which I found this bug has 12 columns, and the
index involves three columns, but the problem is reproducible in a
table with just one column, if that helps to simplify things for you.
I have tried using hash indices and b-tree indices, but in either
case, once you hit the aid values with the larger number of rows, you
get a sequential scan (although the threshold varies slightly depending
on which indexing method you use).

Can this be fixed or is it just a feature of postgres indexing?

Thanks,

Nishad
ps> I can provide the data for the table if it's any help.
--
"Underneath the concrete, the dream is still alive" -- Talking Heads