Inconsistant query plan

Hi,
We are running Postgresql 8.1, and getting dramatically inconsistant results
after running VACUUM ANALYZE. Sometimes after analyzing the database, the
query planner chooses a very efficient plan (15 rows, 4.744 ms), and
sometimes a terrible one (24 rows, 3536.995 ms). Here's the abbreviated
query:

SELECT * FROM t1 INNER JOIN (t2 INNER JOIN (t3 INNER JOIN t4 ON t3.gid =
t4.gid) ON t3.gid = t2.gid) ON t2.eid = t1.eid WHERE ...

In the efficient plan, t2 is joined to t3 & t4 before being joined to t1.
The inefficient plan joins t1 to t2 before joining to the other tables.

We've experimented with different settings, such as shared_buffers &
max_fsm_pages, to no avail. Anybody have a suggestion for getting the
efficient plan to execute consistantly? If you'd like to see the actual
query & query plans let me know.

Best Regards,
Dan