Hello @Andrew_Webb,
Thanks for letting me know your reply to my questions. I agree, setting up the old Macbook Pro will help us eliminate some possible causes for this behavior. While testing the Macbook Pro setup, I would avoid adding more than a few hundred tracks just to test to see if you have the same issue as before. For your question regarding the track load on the Core, Brian discusses the recommended number of tracks & specs a bit here. Please let me know your results with the Macbook Pro when possible.
Thanks,
Noris