Dumb question, but is it possible some data needs to be sanitized, like you're getting something in the string that's whacking the connection itself? Unlikely but not impossible. Once had a Windows program which allowed input without, er, much sanitizing, and a customer sending "X00" would kill the data stream.

Unlikely, but the Internets say:
The source of an ORA-03113 is a broken connection between Client and Server process, a blanket end-of-file on a communication channel error.

...so it seems worthy to rule out?