Could be fuel not getting to the cylinder, or the injection pump worn out so its not delivering fuel to the injectors. It could be the injectors themselves not performing correctly or even the fuel pump or the fuel filter blocked. It could be the head gasket gone so your not getting the correct compression. THere's just a few it could be.
I had a similar problem with a petrol Escort. This was caused by using normal engine oil rather than a semi-synthetic variation. This meant that the hydrolic tappets were getting stuck open or closed and this was causing the mis-fire.