Occasionally slow queries - should I be concerned?

I have a few queries that show up repeatedly in my pgsi/pg_fouine
reports. One of them is a 4-way join SELECT that is called hundreds of
times per day. When I run it myself, it executes in 18-40ms depending
on the query parameters but the logs show a small percentage of those
runs, say 10 times a day, the query runs two orders of magnitude slower
in the 1100-2500ms range.

Is this normal? The box has 16gb of RAM and the database base folder is
12GB in size so I believe the database fits in memory. The
postgresql.conf settings are:

shared_buffers = 4GB
work_mem = 8MB
maintenance_work_mem = 256MB
effective_cache_size = 14GB

Normal behavior I should ignore or worth investigating?

Brian

PS - I do have logging enabled over 200ms and auto_explain enabled over
500ms; not sure if that would factor in here?