I had the same problem. My phasing was so far out I'm surprised the spark was going to the intended post. I used the adjustable rotor to optimize the rotor tips position in relation to the intended cap terminal. I had to use almost all of the available adjustment to get my rotor to point closer to the terminal through out its mechanical advance movement.

I was checking the timing with both a dial back and an MSD timing light. You wont see the multi-sparks at idle because they happen too fast. It just looks like one big spark.

There is a problem with poor phasing. It allows a lot of heat build up due to the large gap the spark was jumping. Mine was burning up my caps and rotors. The heat build up was so bad the metal tip even melted and flew out of the rotor one time, leaving me stranded at Track Day during Spring Fling.

With my phasing issue resolved, I still had erratic rotor positioning. I had already installed the shaft collar to tighten up the free-play between the intermediate shaft and the distributor shaft drive tab. The biggest benefit of that was it dramatically reduced the wear on my drive gear on the intermediate shaft.

Frustrated, I tried locking out my distributor(its easy on a Pro-billet dizzy) and setting my timing to 35*. That completely eliminated the erratic rotor position. It is rock solid at all rpm now. I assume it was my mechanical advance springs allowing the rotor to bounce. I had been using one light and one medium spring.