Power windows on 95 Chevy TravelTime conversion jerks and clicks

I have an 89K mile 95 Chevy TravelTimes conversion van that I just bought. Runs great and just about everything works. But....

The passenger side front window will only go half was down and starts jumping and clicking like it's jumping gear teeth. Going back up works to the top where it does the same thing. What could be causing it to jamb up at the halfway point going down? And why does it keep trying to go up after it reaches the fully closed position?

Any help it greatly appreciated.