It could be several problems. Check the least expensive first and then progress to something more expensive. First of all check the plugs. If the plugs have black soot caked on them then it shows that you have corroded plugs. This could be as a result of plug gapping and condition of the plugs. Check these first. If that is OK then check the plug wires by starting the engine. Then remove the plug one by one and see if you are having an ark. If not, it shows the plug wire as being the problem. If this does not indicate the problem then it will probably be in the cylinder itself which is a major problem of about $600 . Good luck