intermittent ora-12545

intermittent 12545 error which I cannot track down. Pro*c routine.

oracle 9.2.0
solaris SunOS 5.8

since upgrading to oracle 9i from 8i I have had 2 ora-12545 errors 'Connect failed because target host or object does not exist' There is no pattern thus far. It ran for 4 months on oracle 8i without this
error.
The sysadmin said there was little network traffic. The dba said there were
no error log problems.

The Pro*C program connects to prod, inserts one row, then disconnects. It is
part of an asynchronous socket routine and therefore sends one item at a time.
The Pro*C connect is
 EXEC SQL CONNECT :username IDENTIFIED BY :password; where username is a string like "user_at_prod" and password is a string

tnsnames.ora - no sqlnet - the program is on the same machine as the sid
prod =
  (DESCRIPTION =

      (ADDRESS_LIST =
          (ADDRESS =


              (PROTOCOL = TCP)

              (HOST = ourmachine.ourinternaldomain)

              (PORT = 1521)

          )
      )
      (CONNECT_DATA =
          (SERVICE_NAME = prod)
      )