I have had on and off issues with Sonar 64 + Nebula CoreII stability. I'm using version 8.5.3 of Sonar and the latest Nebula version.
The stability has never been great when running a ton of instances (even when not over 50% of the CPU meter is being taxed), so I've changed my workflow to freeze tracks more frequently and that seems to have helped.
That said, some libraries are unfortunately nearly unusable at the moment. The AlexB PRO libraries crash Sonar over 50% of the time when loading...IF it does load, then it's fine, but it crashes it a significant amount of the time, so I've reverted back to the old libraries on that for the time being.