I have a 2006 Nissan Altima, that was misfiring changed the valve cover and gasket with one ordered online and the car smoked like crazy. Replaced it with the old Nissan part it stopped smoking so we bought new Nissan valve cover had a mechanic put it on the car stopped smoking but started missing and coded misfire on port 3 had them change coil pack and new plug. Drove the car until it warmed up stopped and started it again it made a hard start sputtering and idling ruff, check engine light came on brought back to the mechanic and got the same code plug 3 misfiring he changed plug again and it's doing the exact same thing again.drives perfect, idles hard when stopped unless in neutral What could possibly be the problem.