[jpnovak]

Is your mechanical advance in the distributor working properly? You can verify with a timing light at the same time you verify the timing spec.

Have you removed and thoroughly cleaned the carbs? Set the idle balance and idle mixture? Then properly balanced the linkage?

What you describe is classic description of the idle circuit not working properly on the carbs. The transition rpm is a 2600-3000 rpm going from idle to mains. No fuel (or too much fuel) will make them bog and then catch up.

Since the car is unknown and sat for a long period of time I would remove the carbs, carefully disassemble and clean out every passage, float bowl, jet, etc. It will make a huge difference. While you have it apart, verify all the jet, venturi, air correction and emulsion tube sizes to make sure they are appropriate for your car. Just because you think it is original doesn't mean it is after 47 years.