How to reduce PX Deq Credit: Send blkd

Hi,
We have RAC setup with 3 nodes and regularly generate AWR reports on each node for nightly ETL activity.
I am noticing that event 'PX Deq Credit: Send blkd' is top event in all 3 awr report( under the section 'TOP 5 timed events' ).

we have 64GB RAM and 6, 2-core CPUs on IBM AIX machine.

SQL>  show parameter cpu
cpu_count                                  integer     12
parallel_threads_per_cpu             integer     2

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