Any idea what causes this?

First thing, check for vacuum leaks. Don't forget to check the lines inside the car if yours originally had factory A/C.

I had a '73 /6 that acted a lot like yours does. The problem was no accelerator pump. At the time, I couldn't find a carb overhaul kit that included accelerator pump parts. I sprung for a Holley reman that I got through Auto Zone. While it fixed the pump issue, fuel mileage went in the crapper. I opened it up and found it had a huge jet in it. Changed the jet, and it ran flawlessly until I sold it three years later. Part of the point of this is that even the folks who should know better don't always get it right.

To check the accelerator pump, take off the air cleaner, look down the throat of the carb, and quickly manipulate the throttle linkage to wide open. You should be able to see fuel spray in the carb. Repeat this several times and you should be able to smell fuel. Normally, I'd say you can do this with the engine running or not. In the case of this car, IMO it would be better if the carb bowl was full of fuel but the engine not running. I've experienced a backfire to the face...so you don't have to. lol.