Random TNS-12535 Errors during sessions.

 Hi,
We have here an 10.2.0.1 on AIX v5.3. Our main problem is, that some clients report randomly the 12535 Errors. On the server's sqlnet.log, they appearing, too:

Fatal NI connect error 12170.

  VERSION INFORMATION:
        TNS for IBM/AIX RISC System/6000: Version 10.2.0.1.0 - Production
        TCP/IP NT Protocol Adapter for IBM/AIX RISC System/6000: Version 10.2.0.1.0 - Production
        Oracle Bequeath NT Protocol Adapter for IBM/AIX RISC System/6000: Version 10.2.0.1.0 - Production
  Time: 21-JUL-2009 10:59:16
  Tracing not turned on.
  Tns error struct:
    ns main err code: 12535
    TNS-12535: TNS:operation timed out
    ns secondary err code: 12560
    nt main err code: 505
    TNS-00505: Operation timed out
    nt secondary err code: 78