when to increase the value of parameter parallel_execution_message_size ?

We have following DW environment(Oracle 10gR2 on IBM ) :

• 12 CPU
• cpu_per_thread = 2
• pga_aggregate_target > 0

SQL > show parameter parallel

fast_start_parallel_rollback         string   LOW

parallel_adaptive_multi_user         boolean  TRUE

parallel_automatic_tuning            boolean  FALSE

parallel_execution_message_size      integer  2152

parallel_instance_group              string  

parallel_max_servers                 integer  240

parallel_min_percent                 integer  0

parallel_min_servers                 integer  0

parallel_server                      boolean  TRUE

parallel_server_instances            integer  3

parallel_threads_per_cpu             integer  2

recovery_parallelism                 integer  4

during the past many days I am noticing the fluctuating database performance and high value of ' <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";}