Performance Issues with Kontakt

I have a strange Problem with Kontakt 5.0.3 and Nuendo 5.5.3 on OS X 10.6.8. I easily get CPU Spikes using orchestral sample libraries with several dynamic layers. Even one single instrument gives me crackles and spikes. What’s strange about it is that I can use the same samples in Kontakt running inside Vienna Ensemble Pro and everything is fine. Streaming from SSD or regular HD makes no difference and my Mac Pro is pretty powerful 8-Core 2,93GHz with 16 GB RAM. I turned off Multi-Core support inside the VST-Plugin (Kontakt).
Running in 64Bit mode, btw.
So what could be the problem?

Thanks, Andreas