Slant 6 hesitation or stumble

ok so at idle in park or in gear it idles great . When i go to press down on the pedal: gas it stumbles for a quick second or two then picks back up? I've checked everything to specs : factory ect . except for the counter weights sticking and or vaccum advance not working or needing adjusted a little. I believe the vaccum advance works because it starts right up with no hesitation or cranking needed? But under load vaccum advance might a little bit out of adjustment?A little while back i made sure the gap between the rocker arms and valves where properly adjusted running while at full warmed up temps.