kessel

Long time lurker here, ok got my wife's 04 c 230 k ss, 150k miles, been doing most maintenance myself on it since new (except known recalls etc), recently it threw misfire code p 0302 and p 0303 and was sputtering due to misfire. Turn off ignition and back on and misfire is gone and it runs good until you put a load on it (while driving accelerate little faster than usual) and misfire comes back. Now I have another kompressor coupe m271 so I can test various components from it as they are the same engines. I removed ignition coils from her car , ohmed out as per manual( 0.3 to 0.7 ohm ) resistance test and all are good. but i swapped them with other car's coils. reset mil and only code p 0302 came back. After that I removed all spark plugs and visually inspect it , all look same (replaced 7 months ago with NGK iridium plugs) normal , but decide to swap cyl # 2 to position #1. and guess what , the code followed P 0301. Wow I found the issue, so removed the bad plug ran to oreilly auto parts and they gave me 1 free under warranty . Installed it and all good, drove it around for maybe 15 miles and told wife " honey yer car is fixed". the same day both of us gong to local coscto and BAM it happened again, but this time its code p 0302. Removed remainig plugs took back to oreilley and got 3 new , installed reset check eng light , and test the car ... So far so good.Gave back to wife she's happy. Now what are the odds that the spark plug fails in 7 months? Maybe next time I get bosch plugs. Thanks for any input.