In a large saucepan, bring sugar and corn syrup to a boil. Cook and stir until sugar is dissolved; stir in peanut butter. , Remove from the heat. Add cereal and mix well. Press into a greased 13x9-in. pan., In a microwave-safe bowl, melt chips and shortening; stir until smooth. Spread over cereal mixture. Cover and refrigerate for at least 1 hour before cutting.