What style rear feed have chain spkts do you have. Origional had a solid drum, later ones had 3 half shields that float. If the shields do not move freely this is a problem. IF you have the solid drum, as Dan stated I think you have a feeding problem. Make sure there are no setscrews protruding or nicks on the shaft, drum, or shields. If a chain consistantly jumps and looks skewed, it needs to be loosed at the 4 bolts that hold the slats on and rolled loose and then retightened. Worth the effort.