In the engine compartment, check every vacuum line you can get your hands on for obvious aging. These are the small, approximately 1/8" diam., white, maybe black, plastice lines you'll see throughout the engine compartment. Most are traceable eventually to the intake manifold area. Make sure they are all connected to something. If any duck behind the engine, follow with your fingers and make sure they attach somewhere. Gently squeeze/bend with your fingers. They should be stiff but flexible. You'll feel it if one is hard and starts to crack. Remove any covers the lines may go under, especially on top of the engine. Make sure the rubber connectors the vacuum lines mate to are soft with no cracks and the lines go into them securely.

Good Luck, Dan