Sounds like the plastic cable guide on the regulator is shot. Take the door panel off and with the glass in the lowest position, look at the regulator from the top of the door. You will be able to see a white plastic wheel with grooves in it. That is the cable guide. The cable may be off the guide and somewhat mangled. If this is what you find, you'll need a regulator. +/- $120.00
http://www.everythinginfiniti.com Vandi.