I have a motor that prevents the program from running. It simply allows a few slow movements forward with stops in between. It seems very stiff to rotate manually. Any way I can repair this motor?
I agree with HaydenStudios, it is impossible to disassemble completely NXT motor without breaking the orange axle hub. You can nonetheless half-open it enough to see what's happening inside (you need a Torx screwdriver - T10 size if I remember well - to open it. See photos here http://philohome.com/nxtmotor/nxtmotor.htm. Anyway, first try to see what LEGO support can do for you!
I had a similar problem with the RCX about a month ago. Check the wires for shorts or fraying, and if you can open the case and look for shorts in there. I fixed my fraying wire with electrical tape.