Testing three different brand motors this weekend, two of the motors are made by the same company but sold under two different names.
The two motors had the same stutter problem, and the third motor from a different manufacturer was much better, I only had one little stutter with this motor.
There seems to be somewhat of a software glitch with the ESC, the stutter seems to happen more with a motor that has a weaker magnetic.
Let me know if you have experienced this with your PRO.
I don't think the magnet strength has anything to do with it. And from your previous posts, you seemed to have everything in correctly and have tested with different sensor cables and motors. I wouldn't discount the fact that your speedo may be faulty. Send it in to Castle and let them have a go at it....