A rough idle or misfire during partial load could be caused by ignition
sparks jumping from the plug wire connector to the noise suppresion
shield, or the cylinder head.

* Remove and inspect the plug connector for white spots or burn marks.

* On those vehicles equipped with a suppressor shield on the
  distributor cap, it is necessary to remove the shield and check for
  white spots or burn marks on the distributor cap.

if white spots or burn marks are present, replace the damaged
components.