What would cause the engine to miss/shutter from ~45-55 MPH?

This engine has an ignition coil on each of the spark plugs (8) 'Usually' misfire results from one of these coils breaking down and is more pronounced at a certain speed/engine load... Mechanic can use a proffessional scanner set to cylinder contribution test to determine which coil (cylinder) is suspect... Now, it may take some time and test driving to find the problem, so it ain't quite as simple as it sounds!.. I wish it were!!!