my 02 s-type recently started to run rough under light acceleration. i imediately had it sent to the dealership and was told that there was a p0306 stored. i was told that the vehicle needed new plugs so i had them replaced. now i have a p0305 code for a cylinder 5 misfire. The vehicle has progressively gotten worse. it now misfires at idle and seems to bear no relation to any driver inputs. Has anyone come across this problem before?