query taking much longer since Postgres 8.4 upgrade

When I run the following query in Postgres 8.0, it runs in 61,509.372 ms

When I run it in Postgres 8.4, it runs in 397,857.472 ms

Here is the query:

select
course_id AS EXTERNAL_COURSE_KEY,
user_id AS EXTERNAL_PERSON_KEY,
'Student' AS ROLE,
'Y' AS AVAILABLE_IND
from course_user_link
where instructor = false
and course_id in
(
select course_id
from course_control
where to_char(course_begin_date,'YYYYMMDD') IN ( '20100412','20100510','20100607','20100712','20100830','20100927','20101025','20101122','20101213','20110110','20110207','20110307' )
and course_delivery LIKE 'O%'
and course_cross_section IS NULL
)
and user_id not in (select user_id from instr_as_stutemp)

(table instr_as_stutemp has just one column and only 4 rows)

What new feature of Postgres 8.4 would be making the query run so much more slowly? Is there a better way to rewrite the query for 8.4 to make it run faster?

Many thanks,
Julie