Intermittent ORA-1017 error

We are on two node RAC (10.2.0.5)  and I'm getting ORA-1017 error intermittently whenever I try to connect using listener. Whenver I get connection I get on instance 1 only as shown below.

SQL> conn sys/syspassword@pplprod as sysdba
Connected.
SQL> conn sys/syspassword@pplprod as sysdba
Connected.
SQL> conn sys/syspassword@pplprod as sysdba
Connected.
SQL> conn sys/syspassword@pplprod as sysdba
ERROR:
ORA-01017: invalid username/password; logon denied

Warning: You are no longer connected to ORACLE.
SQL> conn sys/syspassword@pplprod as sysdba
Connected.
SQL> select * from v$instance;

INSTANCE_NUMBER INSTANCE_NAME
--------------- ----------------
HOST_NAME
----------------------------------------------------------------
VERSION           STARTUP_T STATUS       PAR    THREAD# ARCHIVE LOG_SWITCH_WAIT