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If the manifold to cylinder head gasket dislodges, then air is drawn into that runner alone, not the others. That leans the mixture and may cause a misfire at idle. However, the additional oxygen is seen by the O2 sensor, and that produces a response by the sensor as a lean condition, and the sensor input then causes the ECU to trim the mixture richer. The ECU can only do that for all cylinders, so the other three end up being excessively rich and the engine will then run very roughly. Mixture is adjusted by dwell time - the number of milliseconds the injector is opened, and to an extent by manifold absolute pressure affecting the fuel pressure regulator.
So the problem should be quickly traced - no fuel from that injector, false air, lack of spark, or lack of compression.
The OP should have enough to go on now.
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