Place ribs on a rack in a large shallow roasting pan. Bake, uncovered, at 350 for 1-1/2 hours. Meanwhile, combine sugars and cornstarch in a medium suacepan. Stir in ketchup, vinegar and water; bring to a boil. Cook and stir until thickened and clear. Remove ribs and rack from pan. Discard fat. Place ribs back in roasting pan; pour about 1-1/2 cups of the sauce over ribs. Bake 30 minutes longer. Cut ribs into serving-size pieces; brush with remaining sauce.