The upper intake manifold gasket and/or a PCV vacuum lines/hoses can develop vacuum leaks with age. This type of leak can result in a higher that normal or rough idle.
Car Problem Reports
Chevrolet Lumina Intake Manifold and PCV Vacuum Lines/Hoses Can Leak Due to Age
Chevrolet Lumina Problem
Engines Affected: 3.1L V6, 3.4L V6