Ok. Your problem is from a poor cleaning of the transfer belt assy or from the fuser.
It will be better if you can attach some pictures here, so we can see and to try to help you. As you sy it could be from fuser or from a drum cartridege. Also be sure to setup the paper settings for paper that you use.
I am printing manuals for my clients that are about 100 pages long. Part way through about 5 or 7 pages will print with smudges. Part of the intentional lettering / graphics do not adhere and the toner then adheres elsewhere on the page where it is not wanted. The color is always back. It is not a repeating mark.
I can attach a scan of the smudged page later today but trying to decide if it is the fuser or the drum. Have printed about 12,000 pages on this printer.
Printer is the Xerox C600 color laser printer.
The only other comment is that I am using inexpensive paper from Wal-Mart (off-brand) rather than name-brand paper. The issue never happened until I started using off-brand paper. Not sure if that is related or concidential, but maybe I will try a ream of name brand paper and see if that fixes it.