Why so much time difference with a same query/plan?

Merry Xmas!

I have a query. It sometimes runs OK and sometimes
horrible. Here is result from explain analyze:

explain analyze
SELECT module, sum(c1) + sum(c2) + sum(c3) + sum(c4)
+ sum(c5) AS "count"
FROM xxx
WHERE created >= ('now'::timestamptz - '1
day'::interval) AND customer_id='158'
AND domain='xyz.com'
GROUP BY module;

There is an index:
Indexes: xxx_idx btree (customer_id, created,
"domain")

Table are regularlly "vacuum full" and reindex and
it has 3 million rows.

QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=139.53..141.65 rows=12 width=30)
(actual time=17623.65..17623.65 rows=0 loops=1)
-> Group (cost=139.53..140.14 rows=121 width=30)
(actual time=17623.64..17623.64 rows=0 loops=1)
-> Sort (cost=139.53..139.83 rows=121
width=30) (actual time=17623.63..17623.63 rows=0
loops=1)
Sort Key: module
-> Index Scan using xxx_idx on xxx
(cost=0.00..135.33 rows=121 width=30) (actual
time=17622.95..17622.95 rows=0 loops=1)
Index Cond: ((customer_id = 158)
AND (created >= '2004-12-02
11:26:22.596656-05'::timestamp with time zone) AND
("domain" = 'xyz.com'::character varying))
Total runtime: 17624.05 msec
(7 rows)

QUERY PLAN

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=142.05..144.21 rows=12 width=30)
(actual time=1314931.09..1314931.09 rows=0 loops=1)
-> Group (cost=142.05..142.66 rows=124 width=30)
(actual time=1314931.08..1314931.08 rows=0 loops=1)
-> Sort (cost=142.05..142.36 rows=124
width=30) (actual time=1314931.08..1314931.08 rows=0
loops=1)
Sort Key: module
-> Index Scan using xxx_idx on xxx
(cost=0.00..137.74 rows=124 width=30) (actual
time=1314930.72..1314930.72 rows=0 loops=1)
Index Cond: ((customer_id = 158)
AND (created >= '2004-12-01
15:21:51.785526-05'::timestamp with time zone) AND
("domain" = 'xyz.com'::character varying))
Total runtime: 1314933.16 msec
(7 rows)

What can I try?

Thanks,

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