Try doing a "stop test." Stop the print process at various stages, and see if which component creates the defect.
For instance, if a print takes 20 seconds, during the first 10 seconds, the drum is generating the image to transfer to the belt. If the image is bad on the drum, then that is your culprit. If not, then see which unit creates the problem.
Also, what color is the line?
After the unit gets to the transfer belt, you should check the transfer roller and fuser.
Whit
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