97 ford aerostar problem?

depending on your maintenance history, start with basics if its only cylinder 5 check the condition of the spark plug wire and spark plug. If its been some time since the plugs, wires, and distributor cap and rotor have been replaced it would be beneficial to change them all. So often the simplest fix is often the correct one, the parts for a "tune up" on your Ford should be around $50-75 from Napa or other retailer, however the labor and hle of working under the hood of any van is not my idea of a relaxing afternoon so if you take it somewhere to be serviced expect the bill to be significantly higher.