slowness when subselect uses DISTINCT

I have several times now run into what seems
like similar performance problems with some 
of my postgresql queries.

I have a view that runs reasonably quicky.

I use this view in a subselect in another 
query and that query too runs reasonably
quicky.

The view returns some unwanted duplicate 
rows so I modify it using either DISTINCT 
or GROUP BY to eliminate them.
View still runs reasonably quickly.

I use the modified view as a subselect as
above, but now the query runs 2-3 orders
of magnitude more slowly than before.

Before I go through the effort of putting 
together a specific and concise test case, 
has anyone seen this general pattern and 
have an explanation or advice?  (PG-8.2.3)