Hellppp code problem

Hello everyone, My 2004 1.4 zr reads code PO300, the light only flashes but doesnt stay on, he runs like a dream, but this code came up when we plugged him in, so we replaced coilpacks, leads and plugs which google and some forum members suggested is what the code means.. yet guess what.. he is still reading it. Misfire randomly over cylinders?.. what else could it be? Ready to give up an cry now x

Thanks in advance

Ems

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