I had to replace the large drive sprocket of a GDO that was mostly worn down. After reassembly, the drive sprocket skips on the drive shaft worm gear when it initially opens and does this pretty much any position of the door. The GDO is probably 20 years old. The door stays open when lifted to waist height and separated from the trolley. Anyone have this issue before? When I replaced the drive sprocket, I bought the assembly with the shaft, cover plate, bearing, and sprockets.