Hello Hugh:
I had a similar problem with my Mac. Martin dedicated a lot of time helping me. Let me share with you some suggestions he sent to me:
-"So I'd just suggest setting the polyphony limit a bit lower on the Mac with the lower performance. If you haven't already done so, disabling all OS X power-saving settings might also help you to get better performance (from any Mac).
-Try putting you old hard-drive back in, just to test whether that exhibits the same issues. If it doesn't then swap your new drive back in, wipe (format) it completely, and and install OS X El Capitan and Hauptwerk v4.2.1 from clean (without importing any OS X settings/data from any other OS X installation).
-Create and send a diagnostic file".
Anyway, my problem was not HW related, but Mac´s .
Luis