Heat oven to 425 degrees F. Spread potatoes evenly in 13 x 9 x 2-inch glass baking dish. Cover tightly with plastic wrap. Microwave on high power for 15 minutes. Carefully uncover. Transfer to large bowl.
Whisk sour cream, cream of chicken soup and 1 soup can of milk in a small bowl. Add to potatoes. Add the uncooked pasta shells, chicken, cheese, salt and pepper. Pour into baking dish. Cover dish tightly with aluminum foil.
Bake in 425 degree F oven for about 45 minutes or until the pasta and potatoes are tender. Let cool slightly and serve.