Sounds like the idle air mixture needs to be adjusted.

Adjust your idle screw untill it idles at a normal speed in park 800rpm maybe. If the engine runs rough toss a rag ontop of the carb (it wont get sucked in dont worry) if the idle smoothes out you probably need to adjust your idle air mixture screws. I am guessing this cars sits a lot so I am suggesting you rebuild your carburator. By that I mean carb kit, float, filter, and a soak in real carburator cleaner.
So called mechanics like to screw around with carbs but dont understand the priciples of operation. That means they cant rebuild them and diagose problems. When you get that thing apart you should find something wrong like secondary throttle plates holding open just a bit or plugged air bleed etc.
Its my opinion that if you have to have a carburator the Quadrajet is about the best all around drivability and performance carb going.
Oh yeah if that thing has bosch platinums in it ditch them!