I just deal with this exact same problem with my '86, though mine aren't power windows. In my case the scraper was somehow bent toward the window (possibly from one-too-many people leaning through the open window?) pushing against the window. It would roll about 2/3 up and then become very difficult. When I tried to roll it down it would stick.
Try rolling the window down and looking from the inside. You'll see the rubber scraper (the part that rests against the outside of the window) right at the top of the outside door frame. Take a flat blade screwdriver and, starting at the front end of the window, hold the flat blade against the rubber on the scraper (screwdriver parallel to the ground) and hit it 2 or 3 times. This will push the scraper away. Move the screwdriver about 1" to the right (toward the rear) and repeat. Keep moving 1" to the right and hitting until you're about halfway across the window. Now try the window.
Thomas