Hmmm, I am by no means an expert, but, it sounds like, from the onset of timing, and vaccum. That type of shudder seems to me, like something is behind.
You rebuilt the quad, did you try a non-vaccum type of carb? Your timing should be all in at that rpm. It sounds almost like it is leaning out at that point.
What is its performance up to, and over that point? Any other symptoms?