lots of "CPU + Wait for CPU" events.

Using Oracle 11.2.0.2, on Redhat.

I have a table T1 with 4 columns
C1 RAW 12 -> PK
C2 VARCHAR2 64
C3 VARCHAR2 25
C4 RAW 12

Also, I have the following indexes

- PK_T1 on T1(C1):  PK (index reversed: ALTER INDEX PK_T1 REBUILD REVERSE STORAGE (FREELISTS 6);
- I1 on T1(C4 )
- I2 on T1(C2 , C3 )

From Oracle reports (ASH,ADDM and AWR ), the following query

        SELECT C1,C2,C3 FROM T1 WHERE C2 = :1 AND C3 = :2 AND C4 IS NULL

causes lots of "CPU + Wait for CPU" events.

Table C1 has 10 million rows with around 15 rows with the same C2 value.

Any ideas to: