Can you share a little more about your build? What's your exhaust configuration? Crossover pipe or none? What's your advance at idle? Idle mixture set right, to the best vacuum assuming you're running a carb or carbs? How do your plugs look - consistent color on all or maybe one that's different? Gary may well be right on an exhaust valve being open a tad, but if you give more details, including what you've done so far someone may home in on the root cause if it's not valve lash.