Performance Tuning and Disk Cache

I have a very slow query that I'm trying to tune.  I think my
performance tuning is being complicated by the system's page cache.

If a run the query after the system has been busy with other tasks
for quite a long time then the query can take up to 8-10 minutes to
complete.  If I then rerun the same query it will complete in a
couple of seconds.

Does anyone know how I can repeatedly run the same query in the
"worst case scenario" of no postgres data in the disk cache (e.g.,
clear the page cache or force it to be ignored)?

Thanks for any help.

Barry