PX Deq Credit: send blkd becomes permanenet part of daily AWR report

Hi,
In our 10gR2 data warehouse environment( with 3 node RAC )  , we see following trend.
Every day in 'Top 5 Timed Events' section , first event is 'PX Credit: send blkd' :

Event                                  Avg Wait(ms)   % Total Call Time
------------------------           ------------         ------------------
PX Deq Credit: send blkd   166                    65.7 ( day 1)
                                           153                   66.6 ( day 2)   
                                           120                   55.6 ( day 3)


We have many queries, which run faster sometimes and sometimes slower. Can the above event have some kind of relationship with unstable performance ?

Best Regards,
Anurag