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.