One thing it might be is that the springs are not evenly tensioned, or tensioned enough. If one spring is pulling more than the other then this could cause the jerking back and forth. I can't be sure of course, because I can't see it. I am assuming that all the tracks are straight and everything is in good shape. I can't stress enough how dangerous it can be to mess with the springs. If you don't know what you are doing you can lose body parts, etc. I'm not going to offer spring instructions... If you know what you are doing, fine. If you aren't sure then seek professional help.