'10 Forester XT - P0304 (after fixing many other problems)

Good Day,

On my 2010 Subaru Forester XT (153KM/95M) I consistently had a P0304 code which was diagnosed by the dealer for a Cracked Piston Ring. An entire new short block, including gaskets, timing belt (was almost due), spark plugs, coolant and more was installed which set me back $9300. Since then I've moved quite to a remote part of the country and about 3000 km later my P0304 has reappeared. This code will turn on, stay on for a day or two (multiple start cycles over that time period) and sometimes extinguish before it comes back a day or two later. I notice the misfire tends to occur as I come to a stop at a stop sign or a red light and the vehicle is idling. Sometimes the engine idles rough and at times I notice my power is reduced but with greater fuel consumption.

I pulled my MAF sensor as it was the only thing I could think of I cleaned it but noticed there is some dried oil on the metal parts that the MAF cleaner could not remove as well as some dried oil stains inside the air filter box. When I had the engine work done the tech had stated that oil was leaking from the engine into other parts so I believe that there has been no oil leak since because prior to the new short block I definitely burned oil but since then I have not lost any oil whatsoever.

Does anyone have any other ideas on what could be wrong? Is replacing my MAF sensor my best bet? Since the coils were good and brand new spark plugs were installed I don't think it could be related to that and since I am quite far from a service station I am unable to take my vehicle in right away for diagnosis.

I very much enjoy my vehicle but these problems are taking quite a hit on my bank account and my patience. Thank you for any and all ideas.