Oracle error messages and possible causes

We have developed an application which connects to an Oracle back-end instance.  The application is a server side service which is always running and acts a middle tier between the client requests and the database.  We have a number of clients who do not experience issues with our software but we have one client who consistently has problems and as a result we added some extra logging to our application recently and caught a number of these errors in our logs.  The application creates an internal pool of connections to the instance and once a successful connection is established it is closed, so we are always selecting from a pool of connections and logging in and executing our SQL.
I have looked at each one and I have done the research on what they mean, but what I am looking for is more of the root causes or possible causes that each of these can occur.  We have done some testing for the last couple of days to try to reproduce the errors by shutting down the Oracle instance, by discionnecting the network cable from the database machine and in all cases the application is able to recover our connections to the database and we cannot get the same messages at all.

Can someone please give me more reasons how these can occur so that I can point my client who is blaming the software in the right direction.

These all occur often and in the middle of our logs while other transactions are being processed.
[Microsoft][ODBC driver for Oracle][Oracle]ORA-01012: not logged on

[Microsoft][ODBC driver for Oracle][Oracle]ORA-03113: end-of-file on communication channel
SQLSTATE : 08S01 Native error code: 3113

[Microsoft][ODBC driver for Oracle]Connection not open
SQLSTATE : 08003 Native error code: 0

[ODBC driver for Oracle][Oracle]ORA-01012: not logged on
SQLSTATE : NA000 Native error code: 1012

Access violation at address 4889131A in module '#39'odbc32.dll'#39'. Write of address 0000001C'

Access violation at address 7C81A379 in module 'ntdll.dll'. Write of address 00000008