I'm with Brian in that it might be the steel gear looking things (pizza wheels) under the print head on the output cover that moves the paper out of the machine. You can probably see from your print just what area it is striking. This also happens with some papers that soak in the ink and get soft leading to the marks, even imprinted tracks on the print's back too. Some papers just don't play well with the 3880 feed system too.
One thing I've noticed with mine is that if the head is too low (Platen gap, Paper thickness, etc.), and the ink is slow to dry on its surface, the head will go over that area and actually rub ink off where those rollers, pizza wheels, gears, or whatever they call them are at. If the gap is too little, it can even scrape off paper fibers to collect in the steel frame that surrounds the actual print head and then you get blobs of ink from those later on.
Trying to clean the bottom of the head is a nightmare, and you can hope the wiper will remove it maybe. I've seen a lot of tarry-looking junk under mine.