simpler query still significantly slower

Heyho,
we tried several versions of a query which give the same results,
but the one takes 5 secs for the first time and 0.9 secs when i
execute the query few moments later, while the other takes ~3 secs
always. How can this be? And how can it be that the second query
which is in fact simpler than the first takes longer?

Query 1: (5 secs vs 0.9 secs)

select
distinct
personen_id
from
produktgruppen
where
produktgruppen.produktgruppen_id in (
select
distinct
r_gruppen_produkte.produktgruppen_id
from
r_gruppen_produkte
where
// this and the next cond are removed in 2nd query
r_gruppen_produkte.gruppen_id = gruppen.gruppen_id
and
gruppen.parent_id=1
and
r_gruppen_produkte.gruppen_id = r_personen_bereiche.g_id
and
r_personen_bereiche.p_id = 1234
)

Query 2: (3 secs)

select
distinct
personen_id
from
produktgruppen
where
produktgruppen.produktgruppen_id in (
select
distinct
r_gruppen_produkte.produktgruppen_id
from
r_gruppen_produkte
where
r_gruppen_produkte.gruppen_id = r_personen_bereiche.g_id
and
r_personen_bereiche.p_id = 1234
)

Our system is Linux 2.2.16, gcc 2.95.2, Postgres 7.0.3 on a Pentium II 450, 128 megs

Thanks in advance

Markus Bertheau
Cenes Data GmbH