I'd say it is fuel related, today's fuel are different compared to the fuel quality that the car was designed for back then, which was heavier.Now they also add butane to make fuel cheaper
I would think that it is percolation, where the gasoline boils in the carb and floods the engine. Does it smell like gasoline when you open the hood ? Also, try pulling the choke if you have one, if it gets worse then it is percolation for sure.