query performance

This is a peoplesoft finance system and the following query is taking varies amounts of time each time it is run, from 20mins to 9hrs sometimes

SELECT OLD.AF_CC_CONSTANT1,OLD.AF_CC_CONSTANT2,OLD.DEPTID,OLD.PROCESS_GROUP,OLD.AF_LIST_VALUE

FROM PS_AF_COSTCNTR_OLD OLD WHERE NOT EXISTS (SELECT 'X' FROM PS_AF_COSTCNTR_NEW NEW WHERE NEW.DEPTID = OLD.DEPTID AND

NEW.PROCESS_GROUP = OLD.PROCESS_GROUP);

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| Id  | Operation                  | Name               | Rows  | Bytes | Cost (%CPU)| Time     |

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|   0 | SELECT STATEMENT           |                    |   992K|    34M|  3953K  (1)| 13:10:43 |

|*  1 |  FILTER                    |                    |       |       |            |          |

|   2 |   TABLE ACCESS STORAGE FULL| PS_AF_COSTCNTR_OLD |   992K|    34M|  1720   (1)| 00:00:21 |

|*  3 |   INDEX SKIP SCAN          | PSAAF_COSTCNTR_NEW |     1 |    13 |     4   (0)| 00:00:01 |