I recently purchased an 05 Envoy with 153000 miles. Ever since I have owned the truck which has only been a week or so it idles roughly. It seems the miss is occurring on cylinders 4 and 6. I have replaced both coils on those cylinders and the plugs in the entire engine. I had a mechanic evaluate the fuel system and he says it is operating perfectly. He suggested replacing the coils and plugs which I did. Once the truck reaches running speed the misses stop and it runs smooth. Any suggestions?