Dear enlightened experts!

Please, sorry for my bad English.
I have decided to try to disturb you by my question.
I have an Oracle running on SCO Unix 3.2v4.2,SCO Unix
We tried to begin to work via SQL*Net2, SQL*Net1 currently in use and
will stay on.
But when Oracle session has been established, it breaks in some
seconds with
ORA 03113 Error "end of file on communication channel". It notices,
that this is
preceded with switching of listener port in status FIN_WAIT2 on a server side, after that any attempt to run sql raise 03113.
We've observed it on all servers, irrespective of operating system,
Oracle versions are all the same.
But I use two machine for diagnostics, with SCO Unix 3.2v4.2 / SCO
Unix 3.2v5.0.5 and Oracle
installed on them. They work correctly, without breaks on SQL*Net2!
All machines are connected to the
same switch, have the similar LAN adapter(Intel UTP 10/100). SCO Unix
3.2v4.2 with SCO TCP/IP release
1.2.1L + net382 + net398b, SCO Unix 3.2v5.0.5 with SCO TCP/IP release

Could you specify me, what to pay attention on?

With respect,