When queries, with a lot of data being send or received, like in ETL or data (up)loads (a la swingbench), suddenly get disconnected, it might be a network issue. There might be no error, or sometimes an ORA-12592…
Application Level Gateway (ALG)
Check ‘layer 7’ settings on your network, one of them being called ALG, Application Level Gateway, which is like a deep packet inspection which tries to get information from packages and change/redirect them based on contents. If the SQL*net protocol or data looks like something ALG recognizes, the packet will be changed and corrupts it.
Different network suppliers might call these technologies differently…