Poor select count(*) performance

Hi there,

I'm sorry for a stupid question but I'm really stuck.

A query:

SELECT COUNT(*) FROM "lingq_card" WHERE "lingq_card"."context_id" = ...;

An hour ago it took 8 seconds, one minute ago the same query took just only 7 milliseconds.

Any ideas why the execution time varies so wildly?

Explain Analyze gives:

Aggregate  (cost=2000.08..2000.09 rows=1 width=0) (actual time=6.962..6.963 rows=1 loops=1)
   ->  Index Scan using lingq_card_context_id on lingq_card  (cost=0.00..1998.68 rows=561 width=0) (actual time=0.025..5.045 rows=2830 loops=1)
         Index Cond: (context_id = 68672)
 Total runtime: 7.011 ms

The lingq_cards table contains about 1.4 million rows.

Thanks,
Mike