each cylender has a coil I believe it is a bad coil.
motor misses out runs rough on 1991 Ford E-150
The surest way to find a misfire is to use an OBD(on board diagnostics)if you have coil over plug ignition. It will tell you which cylender is missing. Swap the coil with one from another cylinder, or a known to be good coil. If the other one starts missing and the first one shows as good then it's the coil. If the bad cyl. keeps missing it's probably the spark plug, I had it happend to me. The plug had a cracked insulator causing it to ground out under load.Replace it. If you have an engine that has three valves per cyl., take it to the dealer or a competent tech. The plugs on those are prone to breakage, you can count on it so save yourself the agravation if you can afford it,you won't be sorry!