Misfiring cylinders 4, 5 and 6

[Nounoon]

Hello Everyone,

I am new to this forum, as I have recently purchased a red 986 2.5 Boxster from 99, with 94k Km.

However, I have some issue with it, and the mechanics I have been to have not helped me up to now, so I turn to the community to help me diagnose the problem.

I have 4 error codes on the car, Random Misfire (P0300), and Random Misfire cylinders 4,5 and 6 (P0304, P0305, P0306). So basically, I have one bank of cylinder that is not working. The car idles with difficulty when cold, and has about 1/3 of the horsepower it should have even hot, but still runs. Even when hot, putting the AC on will stall the engine in idling. When the engine is starting, it randomly makes between 1 and 5 time a big knock, like if you tap two metal parts together, during the first 30 seconds. It also farts dark smoke randomly when cold.

I have changed the Spark Plugs and Ignition Coils without conviction (since it's all 3 cylinders on one bank). I am soon receiving a MAF, but I don't thing it's the issue since it would affect all 6 cylinders.

So I am looking for your experience to help me. I am thinking about injectors (but why 3 on one bank?), air/oil separator (why not all cylinders then?), or head gasket seal?

Do you think that the symptoms indicate one of these problem, or maybe something else that I have not think of ? Maybe one of the thing I listed cannot be the cause and I would waste money changing them now ?

Thank you in advance.
Nounoon