Look for carbon fouling on those plugs. Small bits of carbon that have flaked off somewhere and fused onto the plug’s end. That will cause a high speed miss. Replacing the plugs will only be a temporary fix till they do the same. If this is a lucky guess, then the cure is pull the cylinder head and clean the carbon out. We tend to forget this was a common problem back in "the day". It even was a part of the tune-up procedure spelled out in the old factory service manuals. Today’s oils and unleaded gas have largely eliminated the problem from reoccurring.