When eggs get heated enough, they'll start to scramble. Make sure your pan isn't too hot and/or your egg mixture too cold. Remove pan from heat, and temper egg mixture with reserved pasta water to heat it up a bit more. Tempering your eggs with water, add about 1 Tablespoon of hot reserved pasta (as is in the recipe) to the egg mixture. Whisk together. This should warm it up and help prevent scrambled egg carbonara.