Session Break During Procedure Execution

Hi,we are facing very strange issue. we have a database and our development team have written different procedure used  for multiple purpose. some of them are doing data insert and number of records varies from 5000 to 1500000  and some of them are used for normalization i-e take data from .CSV file and normalize it and then insert into an specified table. 

now problem is since last few days we are facing session termination issue. when we execute to run any procedure it get terminated without writing any error message in alert log. and then we have to rerun the procedure again and same procedure get executed successfully. we are running 10gR2(10.2.0.4) on IBM AIX6.1. one more thing there is a firewall between server and client from where we execute the procedure. and there is also a distance b/w client station and server location about 20KM.