Probably not the problem, but does your Fuel Tank have a vented Cap? When you take the Fuel Cap off to fill the Tank do you hear a vacuum (sucking sound) from the Tank?

When the Engine stops have you checked to see if you have Spark (Heat Soak?)? Do you have a Fuel Pressure Gauge mounted (or have checked) the Fuel Pressure?

Not to say it can't or doesn't happen, but I haven't heard of an icing problem in a gas enigne above 10 or 20 degrees-realize that when youj drive the Temps go down with speed, but I wonder if there isn't a different explanation here-