It doesn't sound like the problem is really in your ball screw. (And that doesn't actually look like a ball screw in your picture; it looks more like a gas spring.) It sounds like you're losing steps, which is a common problem in stepper-based systems. That might be because your gas spring, which is supposed to counter-balance the weight of the spindle assembly, has failed. They don't last forever, but fortunately they are a lot cheaper than ball screws to replace.