Just FYI,
Recently I’ve been using Cubase a lot on my Macpro with 10.6.8, and I’ve never had this problem.
I frequently use plugins like Kontact, Superior Drummer, Garritan GPO4, Yellow Tools Independance, and others.
I’m not using 10.6.3 yet until this current project is finished (call me chicken!) but never had a problem with 6.5.1
I hope you get to the bottom of your problem soon.