Oracle cluster top activity class "cluster"
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.