Question for the brains trust - misfire when not under load.

Hey groovers. My P6B has a misfire that's only present when I'm idling or not under load. I'm still working on my Holley off-idle stumble, but this is different I think. I can drive the car with no dramas - but if I approach the same RPM in neutral there is an intermittent misfire through-out the rev range. I have an old lumenition system for my ignition so I wonder if that's a possibility? What should I look at?