Vibration??

My 69 Dart slant used to do that, for ~15 years. Slowed way down and idled rough in D. Would stumble when leaving a stop light. Had to rev the engine a bit to avoid stalling. Once going, it ran fine. My wife feared driving it. Even changing the whole engine didn't help, plus many other things before that - intake manifold, head rebuild, hoses, electronic ignition, etc. Finally my ~4th carb (Holley 1920) made it run perfect. All the other carbs (rebuilt from Auto Parts) must have idled lean (could never adjust well). I understand the internal metering block of those are sealed, so they can't be rebuilt fully.