My first inclination is corrosion. but I'm not sure how boron corrodes, I know aluminum will leave a white powdery substance behind for example. If its that old it will need to be rebuilt regardless as the suspension can-will decay over time.
one thought is to get a high power magnifying glass or usb microscope and take a look. Look to see how far up it goes on the cantilever if its all the way up into the motor assembly (coil) then its probably going to need a rebuild or possibly its now duff.