Search stops working for Item Object

Search for Item does not work every evening OR whenever Application server (OAS) is re-started. We have two ctx indexes 'ITEM_CTX_IDX' and 'REV_CTX_IDX'. These index remains Valid when search does not work.

I  following sqls return VALID:

select domidx_status,domidx_opstatus,status from user_indexes where index_name='ITEM_CTX_IDX'
select domidx_status,domidx_opstatus,status from user_indexes where index_name='REV_CTX_IDX'

Searches works fine during day time and stop working after 6:30 PST. These is no database process running at that time. This search problem can be resolve temporarily by updating description of any ITEM. Search for ITEM object starts working If I update description of one of my test item and saves it.