Artificer has it said.
Do a tune up and start from there.
Possible causes for a dead cylinder are sticky valves ( comes and goes ), leaky valves ( missing cylinder at idle but runs strong, regular poping noise at exhaust ), manifold leak close to a valve port ( only cylinder out at idle and iregular popping noise at exhaust, cause : broken manifold that might only show when the manifold is removed) , ignition issue. ( faulty spark plug, faulty dist cap, faulty spark plug lead, repro rain caps, humidity inside distributor.)
Blown head gasket, the issue is most present when the engine is hot.
Always start with a tune up and rule out the simple things.
Most can be determined with a vacuum gauge, but one need to be experienced.