I don't have any experience with linked servers, but this might help:
1. You've verified there's only one sqlnet.ora file, right?
2. What does your SQLNET.AUTHENTICATION_SERVICES statement look like:
SQLNET.AUTHENTICATION_SERVICES=NONE or
SQLNET.AUTHENTICATION_SERVICES=(NONE)
3. Using SQLNET.AUTHENTICATION_SERVICES=(NONE,BEQ) might help.
4. Does the SQL Agent have to be in the ORA_DBA group?
5. Make sure the ORACLE_SID parameter is set.