Not sure how helpful this is, it is a timeout, basically.
You mention only happens with larger qrys...
so, can you do a pass-through query to the Oracle db, so the processing is carried out on the server? Same as a normal qry, but when written, right click on the table window of the query design and select SQL Specific --> Pass-through... I'm not a Oracle person though so I would step carefully if not used before though.