Probably what has happened is the spiral cable inside the regulator has been worn down in that spot. Someone may have blocked the window and the gear ground on the cable for a bit in that spot. This is fairly common with this style of window regulator, however usually this problem occurs near the full up or full down positions, as opposed to in the middle.
Something you can try is to remove the regulator from the door, disassemble it, unscrew the spiral cable from the window brackets and then flip it around and screw the other ends of the cable into the brackets and reassemble. (The cable is reversible.) This causes the gear to mesh with a fresh section of cable. With the worn spot in the middle, there is a chance you might still hit that spot, but this fix is free so its worth a shot. A similar thing you can try is to screw the cable in or out a little bit such that you are turning the cable 180 degrees, to cause the opposite side of the cable to mesh with the gear. If doing this, its possible that the cable might spin back to its original orientation, but usually the cable will stay put and not spin. Again, worth a shot before you drop $1K on a new window regulator.
I hope that helps!