I had pretty much the same thing happening to me awhile back with a Q-jet. I spoke with a guy who occassionally posts on this board who goes by the name "carbking". He is very knowledgable about all things carburator--pontiacs in particular.
The way he explained it to me is that todays gas is more volitile than the gas available when these cars were built. The Q-jets float design is such that there is a relatively short distance between the float bulbs and the fulcrum where the weight of the floats rests. When that fuel gets warm, the pressure created in the bowl easily overpowers the weight of the floats, allowing fuel from the bowl to escape through the needle valves down into the IM--flooding the engine. He said the thing to do was to run the starter without pumping the accelerator for a bit when starting when conditions like this were likely to exist. This would have the effect of pumping the excess gas out of the engine until the car would start.
