my 1968 Cougar had windows that were always relatively hard to roll up&down, ever since i bought the car recently. one (front right) was especially hard to move. (previous owners probably neglected this area, maintenance and/or fix.)
because of the more and more power needed to roll this particular window, in a certain occurance, too much power was applied and one center part of the window regulator was broken.
unfortunately, no parts supplier offers that small center part ("drive gear"), but in case of a (multiple-)decade-old part, it actually makes sense to replace the whole window regulator.
to prevent wear & tear, proper maintenance of the window regulators is a "must"... (that is what i will keep in my mind.)