Since the float level puts the gas in the bowl within a fraction of an inch below the venturi, any stop other than a very gentle one can cause gas to slop over into the airstream and create an over-rich idle mixture. Try dropping the float level just a little to allow for some level change during a normal speed stop and see if the situation improves. A leaky float with a bit of gas inside it can also cause problems at idle by letting the float level creep up, but that should also be a problem when standing still, not just when stopping.
Jerry
