Fatal NI connect error 12170 error on AIX

Hi
we have a 10.2.0.2 instant client on AIX connecting to 10.2.0.4 database on Windows

Intermittently we are getting the following in the sqlnet.log file on the client (AIX) machine and the client

Fatal NI connect error 12170.

   VERSION INFORMATION:

     TNS for IBM/AIX RISC System/6000: Version 10.2.0.2.0 - Production

     TCP/IP NT Protocol Adapter for IBM/AIX RISC System/6000: Version 10.2.0.2.0 - Production

  Time: 11-FEB-2013 16:50:35

  Tracing not turned on.

  Tns error struct:

    ns main err code: 12535

    TNS-12535: Message 12535 not found; No message file for product=network, facility=TNS

    ns secondary err code: 12560

    nt main err code: 505

    TNS-00505: Message 505 not found; No message file for product=network, facility=TNS