Radiator reservoir boiling and overflowing

I have a 2006 Subaru Forester that just had a ton of work done on it. New radiator, head gaskets, thermostat, water pump, and timing belt. Yesterday, after driving about an hour, i noticed a pool of antifreeze under the car. When i opened the hood, I saw the reservoir was VERY full, boiling, and spilling over. What could fix this?