Super tuning car runs good shooting for great

I have to wonder if it is:
- A grounding issue
- The MSD is looking for the polarity of the reluctor pulse to be inverted and is presently triggering off of the wrong pulse edge (the slow, falling edge of the pulse)
- Or your timing light is responding to different pulses in each set of sparks. I.e., it triggers off of spark #1 for one cylinder, then triggers off of spark #2 or #3 in the next cylinder's set of sparks. I think MSD has some tech info on timing lights on their site.