Engine misfire increases vibration, lowers performance & gas mileage and loads the catalytic converter with unburned fuel, raising its internal temperatures and shortening its life span. Check the spark plug wires, distributor cap and rotor (on cars with a distributor). Look for hairline (or larger) cracks inside the cap and burns on the plastic portion of the rotor. Replace any parts showing wear. Inspect plug wires for oil soaking or heat cracks on the insulation, and test each plug wire's condition. You can do this with a low-cost tool that substitutes for a spark plug and tests the wire's ability to deliver the spark to the plug while the engine is run during cranking or at idle.
Only purchase high-quality replacement wires. With today's compact engine compartments and high temperatures, only the best insulation can cope with the heat.
The better quality silicone insulation is mechanically fragile, and pliers can easily damage the rubber, so you may want to invest in a set of inexpensive plastic tongs for doing wiring adjustments. Replace a plug wire if you see external deterioration, usually this starts at the terminal ends. Any looseness of the terminal at each end of the wire can easily cause a misfire. Duplicate the original routing of each plug wire, engaging it to its plastic guide, as mis-routing a plug wire can easily cause increased wear and deterioration.
Do not pull a plug wire loose from the plug and hold it next to the manifold while the engine runs. Modern engines are cramped, and the electronically generated spark is higher in voltage than with older cars and the spark may happen to find a better path to ground through you. The spark may damage the expensive ignition box, or may accidentally ignite any oil or fuel residue in the engine compartment.