misfires

mk5. we have rebuilt the turbo upgraded the inter cooler had the computer tuned etc etc. Over the last couple years but now we are both stump. My son kept getting a misfire code on # 4 cylinder so he bought new coils and it stopped the misfire then it started again just on 4 so he took back the coil got another new one and it worked now he is getting misfires on all randomly. I think there is a ground problem. I saw a wire in the loom that had been bent in half but still together. Do you have any ideas or could steer me toward the grounding schematic for the GTI?
Thanks Brian