How to track down inconsistent performance?

We've got some queries that occasionally experience long run times (> 1
minute), and sometimes they're very quick (< 1 second).  Our theory is that
when the queries are fast it's because all the "right stuff" is already in
memory, but we don't know how to actually prove that. (We think we've
successfully ruled out disk contention -- iostat shows disk activity during
these queries, but nothing that's excessive.)

We've turned on statistics collection-- does anybody have any hints about
what things we should specifically be looking for?

Here's what explain says:
bash-2.05$ time psql quickview -c "explain select distinct
os,compiler,stdlibtype,threadlib from builds where product='sourcepro_db'
and visible=true order by 1 asc;"
NOTICE:  QUERY PLAN:

Unique  (cost=197093.74..197588.02 rows=4943 width=50)
  ->  Sort  (cost=197093.74..197093.74 rows=49428 width=50)
        ->  Index Scan using builds_visible_product on builds
(cost=0.00..192225.34 rows=49428 width=50)

quickview=> \d builds
                      Table "builds"
      Column       |           Type           | Modifiers
-------------------+--------------------------+-----------
 id                | integer                  | not null
 visible           | boolean                  |
 state             | character(1)             |
 evaluated         | boolean                  |
 product           | character varying(30)    |
 compiler          | character varying(30)    |
 os                | character varying(30)    |
 stdlibtype        | character varying(30)    |
 linktype          | character varying(30)    |
 threadlib         | character varying(30)    |
 exportlevel       | character varying(30)    |
 usermode          | character varying(30)    |
 postbuildclean    | character varying(30)    |
 prebuildclean     | character varying(30)    |
 submitted         | timestamp with time zone |
 started           | timestamp with time zone |
 finished          | timestamp with time zone |
 machine           | character varying(100)   |
 errors            | integer                  |
 warnings          | integer                  |
 testsattempted    | integer                  |
 testspassed       | integer                  |
 testsfailed       | integer                  |
 examplesattempted | integer                  |
 examplespassed    | integer                  |
 examplesfailed    | integer                  |
 ping              | timestamp with time zone |
 start_count       | integer                  |
 user1             | character varying(50)    |
 user2             | character varying(50)    |
 user3             | character varying(50)    |
 user4             | character varying(50)    |
 user5             | character varying(50)    |
 user6             | character varying(50)    |
 debug             | character varying(30)    |
Indexes: builds_compiler,
         builds_compiler_finished,
         builds_compiler_state,
         builds_compiler_submitted,
         builds_machine,
         builds_machine_finished,
         builds_machine_state,
         builds_machine_submitted,
         builds_os,
         builds_os_finished,
         builds_os_state,
         builds_os_submitted,
         builds_ping_finished,
         builds_pr_os_comp_std_thr_u1,
         builds_product,
         builds_product_finished,
         builds_product_state,
         builds_product_submitted,
         builds_started,
         builds_state,
         builds_visible_finished_product,
         builds_visible_product
Primary key: builds_pkey
Triggers: RI_ConstraintTrigger_25192277,
          RI_ConstraintTrigger_25192279