I tried to make a Quattro Formaggi (4 cheese sauce) the other day using asiago, parmesan, ricotta, and mozzarella. I melted 1/2 a stick of butter in a large sauce pan, then added all of the cheese and some milk to thin it out. When the pasta was finished I combined it with the cheese sauce and baked for 30 minutes. Initially it was good and gooey, but as it cooled it started to coagulate and get lumpy. Does anyone have a suggestion so that when I make it again it stays creamy?