Our 87 300D Turbo had the same sluggish sunroof problem. After replacing the side lift rails (both sides) replacing the cable to the motor in the trunk and then taking it back a fourth time, the mechanic discovered the guide tubes for the cable had opened up (like a split lengthwise. The cable was getting caught in the open part of the tube
The repair includes removing the rear window, so we have not yet had this fixed. I really miss my sunroof, though.