I can't give a definate answer but I would think it is the fuser, it looks like it is damaged, and when it heats up (mid run) it softens and the repeating degradation goes away, only to return when cooled.
Please be sure to select "Accept Solution" and or select the thumbs up icon to enter Kudos for posts that resolve your issues. Your feedback counts!
Our Xerox Phaser 6180 has started streaking with black liines when printing. However, if you print a large quantity the lines goe away but only for that print job. A new job will have the lines return.