It could be a bad channel on the cartridge, could be a broken lead in the arm/cart wiring. Try swapping the right and left hot leads (at the cart) and see if it moves, then the right and left ground leads. If either causes the bad channel to move, then the problem is in the wiring. As you can probably tell, that wiring is pretty fragile. If not, then it is in the cart. If it is the cart, then you may have to do some gymnastics to figure out if the stylus or the body is bad.