Oracle cluster top activity class "cluster" When under high load (many queries in parallel) our Oracle RAC gets in a state where most of the activity is reported as "cluster" by Enterprise Manager.  It stays like that for 30 minutes to a couple of hours, and then just recovers by itself and happily proceeds.

Sometimes instead of "cluster" the activity is reported as "configuration" or "concurrency". 

I'm looking for any advice on how to go about diagnosing what is going on (and how to make it stop doing it). The graph shows only one node in question.  Queries are run on a service which is limited to that node.
Investigate v$session view about what particular event is causing those cluster waits. E.g.:

select s.event, s.* from v$session s order by s.event;


event column will get you offending event event name. Then you can google the event or add information to you question.
Also "Blocking Sessions" screen in Performance menu in Enterprise Manager in such cases helps quite often. Some session may simply block other sessions.