The first thing I would suspect is a plug wire that didn't get fully-seated. That will lead to arcing between the terminal and the spark plug, and eventually the wire will burn back far enough to cause a misfire. Don't overlook the distributor cap and rotor. Check for carbon-tracking, cracks, and moisture.
Normally misfires will be detected by the Engine Computer which will set a diagnostic fault code specifying which cylinder is at fault. When no code is set, consider other causes of harshness such as a collapsed rubber isolator in an engine mount.
Tuesday, May 12th, 2015 AT 6:25 PM