The lean mixture at cruise can overheat the plugs which at WOT then preignite the mixture. Then as the plugs cool off it runs normal.
Check your plug color at cruise. If they are white to yellow, they are running too hot. If the insulators are speckled, that may be evidence of past preignition. If the porcelains are glazed, replace with lower heatrange plugs, and fix the AFR.
The long-standing hesitation points to a momentary lean AFR. This is easily fixed but the how of it varies with carb type, coolant temp, and Ignition timing is a factor.
Make sure your vacuum advance is working correctly
