saving large number of nodes

I am trying to save around 1000 product nodes. before saving I am fetching these record from external system and processing in scheduler. tryed saving node one by one after processing completes for one node, session was loosing data in between and one node has lot of data. So trying multiple times session.save for one node data after certain amount of precessing is done.

so with this approach getting a error "This session has been closed" randomly. Some time it is working but not always.

Any suggestions on this?