Too lean at idle could be your issue. Have you tried adding a little fuel to see if it changes anything? Do you have a wide band to measure the o2?
Today's fuel lights off way fast and is designed for fuel injection, so in this heat (cooler than it has been) you might be getting some vapor lock for bowl boil.
Other thoughts, do you have vacuum advance hooked up? If so is it to a ported vacuum source or full vacuum? It should be connected to ported. What is the mechanical advance in the distributor?
