I had the exact same symptoms, if yours only does this when fully warmed up.
I threw 5 gallons of 100 octane race fuel in the tank and the problem appears to have gone away. (Mixed with half a tank of amoco ultra I bought mid february)
I think it's due to winter gas formulations causing the fuel in the bowl to boil.
If you search for winter fuel on here you'll see some pretty compelling threads to support this.
Like this one:
winter fuel formulation
Chuck