How can I tell if my NXT motor is faulty?
If the one motor exhibits the same behaviour on any port with any cable and the other motors are working on all of the ports with any cable, then it is safe to say that the problem is definitely with the motor itself.

Make sure you swap cables as well as motors to rule out a faulty cable.

Also, check for signs of corrosion on the pins of the cables, motors and NXT. This kind of erratic behaviour is indicative of poor electrical connections.