I was going to suggest plug wires except for it smoothing out at higher rpm. But it is certainly worth a look. Shortly after I bought my Trooper a cylinder started missing, drove home from a long trip on 5 cylinders (long story). When I pulled the plug wires there was a huge carbon track on one of the plug tubes. New plug wires cured that.

Sometimes you can run it in the dark and look for arcs, but the way the plugs are buried in the valve covers the arcing can be hidden.

Steve Carlson - 95 Trooper LS expo rig
Serenity now!