Get a mechanics stethescope and listen for arcing in each of the coils. If you hear any, it would be suspect. Or try swapping them out one at a time with a known good one to eliminate a coil.
Check to be sure the plug wires are good and tight and snapped on to the plug. Check for carbon tracking down the side of the plug. It may appear as a cracked insulator.
Be sure the wires are routed correctly.
Use the stethescope it listen to each individual injector when it is running rough. You will hear it clicking as it opens and closes. A silent one would be suspect.