2006 Corsa Twinport Misfiring Problem

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I have a 2006 Corsa Twin Port. Over the last two month the car has started to intermittently misfire initially on 1 cylinder now on two cylinders. When it starts to misfire the engine management warning light appears.
A full diagnostic check carried out, only fault reported is the cylinder misfire. The coil pack has been changed, injectors swapped over and ultimately the ECU replaced but the car still intermittently misfires.
Does anyone have any suggestions to what the root cause could be and how to rectify the fault??