Alright, maybe we need to locate another one of those check ball assys to start narrowing this down. Tho I am thinking that it there was no vac leak that a warmed up eng would only have a flat spot/hesitation if the AP was non op/inadequate (part or full time) & not be coughing back thru the carb but I ain't 100% sure on that. EDIT one of my fav tricks for this is to block the rods up & take her out for a cruise & see what being on the power circuit does to its behaviour