On 98's that have seen a lot of use you get ink and general muck buildup on and around the heads that is not removed well by normal cleaning, after while it can start marking the prints.
One solution is to moisten a paper towel, fold it in thirds. Fold an 8.5x11 sheet of paper in half as a thin support, wrap about 1" of the folded paper around the back of the paper support, with the pressure bar released shove the towel + sheet down into the paper path, towel side out. Oh, the printer should be powered down. Engage the presssure lever. Now press down on the blue thingy near the heads to release the carriage and drag it back and forth across the towel a few times. What a lot of gunk! Release the bar and push the towel a little further in. Release the lever, push the towel in a little more, repeat dragging, more gunk! Keep going until the black gunk starts to give way to colored ink. Usually takes one towel change for severe cases. Needs some practice not to snag the towel which should be folded neatly and flat. Also clears mega-clogs pretty well. Also check your Paper Thickness setting especially if you are using thickish paper.