Inconsistent query performance

Hi all,

I'm having issues with slow queries using postgres, and I'm finding some of the issues difficult to reproduce. My
applicationlogs slow queries for me, but often, when I go back to run explain analyze on the query it is very fast. I
assumethis is due to some sort of caching. Here is an example: 

sqlLogging _valuesForSQL ran 116509ms with sql string:

select t1.transaction_id, t1.campaign_definition_id, t1.campaign_detail_number, t2.name as campaign_name,
t1.communication_type,t1.delivery_error, t1.call_result into temporary activity_transactions_temp from
customer.customer_transactionst1, campaign.campaign_definitions t2 where t1.customer_id = 2577 and t1.timestamp >=
'04/08/2013'and t1.timestamp < '04/09/2013'  and t1.campaign_definition_id = 23   and t1.campaign_definition_id =
t2.campaign_definition_idorder by campaign_name, communication_type; 

But if I run an explain analyze on that I get

"Sort  (cost=18.08..18.08 rows=1 width=89) (actual time=767.051..767.061 rows=17 loops=1)"
"  Sort Key: t2.name, t1.communication_type"
"  Sort Method:  quicksort  Memory: 26kB"
"  ->  Nested Loop  (cost=0.00..18.07 rows=1 width=89) (actual time=758.421..766.881 rows=17 loops=1)"
"        ->  Index Scan using timestamp_only_ndx on customer_transactions t1  (cost=0.00..9.79 rows=1 width=69) (actual
time=79.771..88.119rows=17 loops=1)" 
"              Index Cond: (("timestamp" >= '2013-04-08 00:00:00'::timestamp without time zone) AND ("timestamp" <
'2013-04-0900:00:00'::timestamp without time zone))" 
"              Filter: ((customer_id = 2577) AND (campaign_definition_id = 23))"
"        ->  Index Scan using campaign_definitions_campaign_definition_id_key on campaign_definitions t2
(cost=0.00..8.27rows=1 width=24) (actual time=39.922..39.923 rows=1 loops=17)" 
"              Index Cond: (t2.campaign_definition_id = 23)"
"Total runtime: 770.830 ms"

Is there anything that can tell me why this query took 117 seconds, and what, if anything, can be done to fix it?

Here's another much more simple one:

expression took 20147 ms:

SELECT t0.appointment_id, t0.customer_id, t0.event_date, t0.patient_id, t0.transaction_id FROM
customer.customer_transactions_detailt0 WHERE (t0.patient_id = 7441 AND t0.customer_id = 2965) 

"Index Scan using customer_id_patient_id_idx on customer_transactions_detail t0  (cost=0.00..10.22 rows=1 width=24)
(actualtime=35.952..99.487 rows=14 loops=1)" 
"  Index Cond: ((customer_id = 2965) AND (patient_id = 7441))"
"Total runtime: 99.537 ms"

So it took 20 seconds at 12:18pm today, but now it takes ~100ms.

Thanks,

Ramsey