If like slimbob says its on the BACK of a single sided copy it cant be coming from the PCU then can it since that only touches the front side of a copy.
Either its the Paper Transfer roller is full of dev or the Registration section as some in it also its probably worth replacing the erase lamp assembly because if that's not working it will cause the image that it was supposed to be erasing getting onto the transfer roller before you press the green button.
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