The 235 I had in my '49 when I first got it used to do the same thing. If it was close to freezing and raining/snowing I could count on it trying to die at the first couple of stop signs.
As soon as I shut the motor off the first time (like parking to run into a store) then it would be fine. Used to find frost all over the insulator between the carb and manifold. Never did figure out a good explanation for it. It had the stock intake and exhaust setup and a working heat riser valve.
