As you have changed the plug and injector (or swapped them around)

It COULD be a fucked plug lead or dizzi cap causing a missfire....


OR... ( along shot)

I had a similar issue on my ecu but that was caused by the crank sensor
and/or the phase sensor miss adjusted and also damage on the crank
pulley and causeing the ecu to think the car was continuously starting
causing the first injector to fire twice. (This was on a P8)