'70 318 BBD falls flat under load

More clues. I took it out on the rural two lane highway our road is off of. If I ease into it, once the throttle is open and I'm accelerating, it takes off like never before. But if I get off the gas and punch it, it stumbles and when it catches itself it takes off again like a new engine. If I just partially let off on the gas and punch it, it takes off great. It's only if I release it all the way and try to accelerate at anything other than easing it open. Again once opened up I can punch it and go.