Friend of mine had the same problem, didnt chnage much but went too a digital 7 and the HVC 2 coil, he gapped his plugs too .040 and it starting breaking up at high RPM, we checked alot of little stuff... The Ign box, timing, pickup in the dizzy, turned out, when we gapped the plugs to .030 the issue went away. Only thing we could comeup with is that the larger bore size, 4.505, it was alot of windage in the combustion chamber blowing out the spark at 6500RPM plus. Hope this helps!