Check the distributor and see if it is cracked or the electrodes underneath the cap are burned or corroded, if so,Replace with a new one.Check the rotor as well, replace it, if it is worn.The spark plug wires should be inspected for cracks, and look for bare spots with the wire coming through, if they are oil or gasoline soaked and the insulation on the wires is soft,Replace them with new ones.... Check the #3 and #4 spark plugs, see if they are oil fouled, or gasoline fouled,If they seem to be oil fouled, you may have a worn out vavle guide or valve guide seal... If they are gasoline fouled, you may have too rich a fuel mixture at idle, or when you first crank the engine when it is cold,too much fuel is sent to the cylinders, then the 'cold start solenoid' may be the cause.You might have to switch to a hotter type of spark plug,The factory cold type spark plugs should be substituted for a hotter CHAMPION SPARK PLUG, or MOTORCRAFT SPARK PLUG.