Same problems here.
I was blaming Oracle 11g, but seens like a bug in plsql developer. I've read the entire forum for an answer. This is really annoying.
PL/SQL Dev. version: 9.0.4.1644 01.493953
Oracle Client: 11.2.0 Windows XP 32bit
Oracle Server: 11.2.0
The same thing is happening to me. I did the modifications you suggested but the aplicattion still crash after I try to break/interrupt some debug session.