Problems with genetic optimizer

Hi all,
I generated a table with a column type 'box', inserted some values and
created an index (ops_box) on that column, run a vacuum and looked with
the command EXPLAIN if the select will use the index.
And the result was very astonishing:
if I had inserted 120000 elements postgres will _NOT_ use the index, if
I had inserted 100000 elements postgres will use the index!
When I set the sequential search off ('set enable_seqscan off;')
postgres is using the index in both cases.
Has someone any idea what to do?
Peter

--
        Bezirksfinanzdirektion Muenchen
              Vermessungsabteilung
.........................................................
 Peter Keller        :  Tel: (+49) 089-2190-2594
 Vermessungsrat     :  Fax: (+49) 089-2190-2459
 Alexandrastr. 3    :  mailto:Peter.Keller@bvv.bayern.de
 80538 Muenchen        :  web: http://www.bayern.de/vermessung