I had a misfire problem for the past month after installing a new ECU, cam gears and injectors (R32 GTR). We replaced ignitor, coils, plugs, ECU - checked fuel filter, fuel pump, earthings etc etc - remapped fuel also. After doing this (all on Dyno etc) we still couldnt resolve it.
IT ENDED UP BEING THE EXHAUST MANIFOLD GASKET!!!
If you get nowhere on the electrics try to listen out for a hissing noise around the exhaust manifold.