Oracle Query Times out and or Object No Longer Exists

Question

wondering if someone can help me streamline the below query. It will run for hours, and then it give idle timeout warning. We updated the idle time to unlimited and now we get: Error message: ORA-12801: error signaled in parallel query server P007 ORA-08103: object no longer exists

Query:

SELECT proj_id,
       wbs_id,
       task_id,
       rsrc_id,
       taskrsrc_id,
       SUM(commit_amt) commit_amt,
       SUM(oblig_amt) oblig_amt
FROM (
SELECT   pt.proj_id,
         pt.wbs_id,
         pt.task_id,
         ptr.rsrc_id,
         pli.taskrsrc_id,
         SUM(NVL(pli.certified_us_amt,0)) 
       + SUM(NVL(pli.obli_excess_commit_amt,0)) 
       - SUM(NVL(pli.deob_amt,0)) commit_amt,
         CASE WHEN pli.moa_code LIKE 'I%' THEN SUM(NVL(pli.certified_us_amt,0)) 
                                             + SUM(NVL(pli.obli_excess_commit_amt,0)) 
                                             - SUM(NVL(pli.deob_amt,0)) 
                                             - SUM(NVL(pli.unoblig_us_bal_amt,0))
              ELSE SUM(NVL(oli.oli_approved_amt,0)) 
         END AS oblig_amt
FROM     pr_line_item pli
LEFT OUTER JOIN obligation_line_item oli ON (pli.etl_foa_code = oli.etl_foa_code
                                                  AND pli.prac_no = oli.prac_no
                                                  AND pli.prac_line_no = oli.prac_line_no)
INNER JOIN resource_codes rc ON (pli.etl_foa_code = rc.etl_foa_code
                                      AND pli.resource_code = rc.resource_code)
LEFT OUTER reorg_xref prx ON (prx.old_org_code = pli.labor_receive_org_code)
INNER JOIN taskrsrc ptr ON (ptr.taskrsrc_id = pli.taskrsrc_id)
INNER JOIN task pt ON (ptr.task_id = pt.task_id)
WHERE rownum < 100 and NVL(pli.taskrsrc_id,-1) > 0
AND NVL(pli.certified_us_amt,0) + NVL(pli.obli_excess_commit_amt,0) - NVL(pli.deob_amt,0) > 0
GROUP BY pt.proj_id,
         pt.wbs_id,
         pt.task_id,
         ptr.rsrc_id,
         pli.taskrsrc_id,
         pli.moa_code
)
GROUP BY proj_id,
         wbs_id,
         task_id,
         rsrc_id,
         taskrsrc_id;