I wisk it was easy to tell. I'm pretty sure it is the jack now. The computer is only about a year old and I've already been through a couple of cords. (One time thanks to my kitten though, so that doesn't count lol!)
Push jack in and out ten times to rub off any possible oxidation. Might do the trick, but I think it's the laptop's jack. Interesting how others have had jack problems and needed soldering and repairs