Yes as U said,

If on one side a query is executed and simultaneously a DML is happening at the same time.  THEN the user doing the DML action gives a commit in between while the query is still executing on the other side then, on the query side U get this message "SNAPSHOT TOO OLD"

Suggestion :

Alter system set UNDO_retention = 900 ;

or give a large value for undo retention, ie., the undo block will be kept in the memory until 900 microseconds even after the commit transaction on the same table, so that the query ends within that.