Hmmm, I'm no expert here but I'm learning. I'm not sure if you have a stock carb. or not, but a leaky choke pull off could possibly be a cause. If it's leaking, then it may take a few seconds for your engine/carb to have enough available vacuum to keep the choke open. Does the car seem to run rich at all? Black smoke or fuel smelling exhaust? I would think that whatever is causing it is also causing a rich condition, at least for part of the time. Give us some more info. maybe. Stock Q-jet or stock Holley, or aftermarket carb, mods, etc.?