When you are getting lots of ORA-3136 in RAC 10.2 then the following might help. You need to edit sqlnet.ora in the $ASM_HOME or $ORACLE_HOME, depending on where the listeners are started from. Add these keys in addition to what you defined in DEV/Test and if your problems disappear, promote it to production.

SQLNET.EXPIRE_TIME =10
INBOUND_CONNECT_TIMEOUT_LISTENER=0
SQLNET.INBOUND_CONNECT_TIMEOUT = 300
SQLNET.SEND_TIMEOUT = 300
SQLNET.RECV_TIMEOUT = 300