IMHO it sounds to me as if it's more to do with transitioning from one fueling system to another. By chance are you still running power jets? One of the reasons for blanking the power jets is that it can be difficult to tune with both factors at play. As there are lots of wide open spaces in Oz, I toyed with retaining the PJ's to provide extra fueling on long straights, and allow me to run smaller mains. I ran smaller mains and PJ's with c needles and had a similar issue to what you are describing. In the end I have returned to bigger mains, no PJ's and B needles. it runs a little rich in the middle, but gives me more confidence at wot, and there are no stutters.