SORT ORDER BY

Hi,
In the database 11.2.0.2

I have the following query :-

SELECT /*+ INDEX( cro_1_053 IX_CRO_EXTR_1_R9_BIS)*/ cod_pays FROM CRO_1_053 WHERE num_gen = 405 AND nom_fich = 'GDG' ORDER BY date_heure_cro,champ_perso_0;
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| Id  | Operation                   | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
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|   0 | SELECT STATEMENT            |                      |     1 |    27 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| CRO_1_R9             |     1 |    27 |     3   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IX_CRO_EXTR_1_R9_BIS |   904 |       |     2   (0)| 00:00:01 |
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This is the GOOD execution plan,now sometimes i get this execution plan as below which is BAD :-

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| Id  | Operation                    | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
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|   0 | SELECT STATEMENT             |                      |   904 | 24408 |   641   (1)| 00:00:08 |
|   1 |  SORT ORDER BY               |                      |   904 | 24408 |   641   (1)| 00:00:08 |
|   2 |   TABLE ACCESS BY INDEX ROWID| CRO_1_R9             |   904 | 24408 |   640   (1)| 00:00:08 |