Cursor Problem

I have a cursor defined thusly:

Cursor ResultCur (nMinTestResultHistoryID Number) Is

    Select *
    From ATT.Per_Test_Rslt_Hist
    Where Num < nMinTestResultHistoryID     Order By Num Desc;

The where clause is causing me to blow out my temp tablespace even though the Num field is indexed. If I take the where clause out the rest of the stored proc starts running right away. The table contains 1.6 million records.

The index in Num is defined thusly:

CREATE UNIQUE INDEX ATT.PK_PER_TEST_RSLT_HIST     ON ATT.PER_TEST_RSLT_HIST(NUM ASC)
PCTFREE 10
INITRANS 2
MAXTRANS 255
TABLESPACE ATT_DATA
STORAGE(INITIAL 1M

        NEXT 19928K
        MINEXTENTS 2
        MAXEXTENTS 249
        PCTINCREASE 50
        FREELISTS 1
        FREELIST GROUPS 1)