I had a problems like that when I had a 1962 Fiat 600D. For what it is worth, here goes. It could be the spark plug or the HT wire. Try switching the HT wires to see if one of them is faulty. If both are okay. It could be the spark plug, but when was the last time you changed the plugs? Remove the spark plug in the problem cylinder. Fouling is a common cause of ignition misfire. Examine the end of the plug. If the electrode is covered with deposits, clean or replace the spark plug. Also, note the type of deposits on the plug. Hopefully, the deposits on the plug is not thick, black, wet or oily-looking deposits. Try switching the plugs as well. It could be the distibutor cap. Inspect the cap for cracks. If all the aforementioned are good, it could be the ignition coil.