Help

Hi new to mini coopers....having a car starting problem. Changed plugs after it threw me some codes ...it seems to stutter and not start on the first coupla trys..seemed to get better after plug swap, but it's back and this time there's no codes or check engine e light.....it stutters and shudders then levels out after a sec....runs fine until I turn it off .. it's not everytime but happens alot...any help as to why???