MacDude,
A couple of questions:
Can you tell us what MacOs versions you are running on each of your MacPro's?
Does this issue happen when using Kontakt 6 as a VST or an AU?
Just to clarify, you've tried disabling the multithreading option in the Kontakt 6 and letting VEP 6 manage all mulithreading and the problem still occurs?
Is your Kontakt Memory Server Disabled or Enabled?
Lastly and least relevant, is this a custom patch or from a comercially available library? If the latter, which Library/Patch. Just curious.
Answering these questions will help me further diagnose. Also, any screen shots of your settings would be helpful.
Best,