There were a few different management systems on this car. By your description, I am guessing your car is between a 1990 and 1994 or so. These had a switch in the inside of each door that was closed by the window hitting the bottom of it's travel. If one of these switches didn't operate correctly, the top would operate as you state-it would stop in the vertical position, because the control module needs to see the windows in the full down position before going any further than the vertical position, or it could damage one of the windows. My guess is that the part of the window frame that contacts the switch has come out of adjustment.
When the top stops in mid travel, try gently pushing down with your hand on the top edge of the glass, on both sides, to see if the top will start operating again. Try it one side at a time to see if you can locate the side that is causing the problem.

Gilly