Could be something really simple, like a dead spark plug, or an HT lead that has reached the end of it's life, they are silicone inside and can break internally, although they look fine on the outside. Something a good service will cure.
Broken Spark plug, loose spark plug, loose or damaged HT lead, EMU problems, Cylinder head gasket damage, piston compression ring leaking, burnt out valve, cam shaft lobes worn, take your pick!! Best way of finding out is to take your car to a garage with an engine analyser they should be able to diagnose the problem.