I had the same problem last year random misfire and sometimes no spark or just a weak spark would break down one day and the other day would be fine I changed the complete ignition system fuelling systems sensors you name it after a couple of months of head scratching I looked under my dash to out find my heater matrix was leaking anti freeze into my ecu! so check your ecu and also if you have an fcd that can also issues if faulty aswell hope this helps good luck