Maybe your float bowl is going dry! That was my problem. It was interesting, when I bought the car the owner had installed an electric fuel pump which he ran for a bit before even attempting to start the car. Basically what was happening was the pump was refilling the float bowl.
If you open up early rochesters you will find they have a couple of plugs in the bottom of the float bowl. As you can see from the picture mine had already been repaired, but not good enough!
There was also evidence of the carb being run without the stainless plate that goes between the carb and the manifold.