Tom,
When one of your axis stalls, you will hear a screaming noise coming from the stepper. The axis position will continue to show a change on your screen but the axis will remain stationary, with the stepper making a loud racket. As balsaman says, an assist spring is almost a sure fire way to cure this if it’s due to a heavy Z axis. You might also want to reduce the acceleration speed and check for any binding in the travel when the stepper is physically disconnected from the drive screw.
Steve
www.tac-pro.com