Engine sputtering at startup, code P301

Cold or hot, 90% of the time when I start the engine, it is sputtering, as if one or two spark plugs do not work. But after around thirty seconds all is okay, power okay, driving okay, no power loss. The misfire code P301 comes up, a mechanic suggested to change all the spark plugs with something of good quality, even though I changed them all few months ago (from Canadian tire). Other than bad idle at startup, all is okay. When the startup is bad, I feel in the cabin bad emissions, they go away when the engine comes to normal, after about thirty seconds from engine running.