I think your fuser roller may be dirty or damaged. Try printing several pages that are blank or nearly blank. If it's dirty, the lines should get progressively lighter. If not you may need a new fuser.
You can buy a fuser repair kit for your printer model online. They are usually only slightly more complicated than changing out the toner cartridge. Just be sure to lay down several sheets of printer paper to cover your work area to catch any toner dust.