On a Stg 3 Cossie 4x4, I had misfires on boost, and slightly on startup when cold. Bought a Group A coil and fitted it and the misfires were gone, but after about only 10 miles driving I was back where I started, with misfire and total power loss around 4k RPM.

Could the coil have failed that quickly? Or what else might be going on?

NB I tried new plugs and leads too but that made no difference.