Hi guys car has just started to develop a shudder when I pull up at junctions traffic lights etc doesn't happen all the time when I take off throttle unresponsive for a second or two then kicks in have reset throttle response no good sometimes feels as if car is going to cut out has been suggested could be a faulty spark plug any thoughts I have a 2007 roadster with 75,000 Km recently changed oil filter ,oil air filters but doubt that's the cause