Stepper Motor juddering and reversing direction

Hi

I am having a strange issue with one of my Nema 17 z motors (the end that carries the X axis motor)

After working fine for about 3 hours, it will now only judder or move in in 1 or 2 1/4 rotations before reversing the direction.

I am using Ramps 1.4, Sprinter firmware and pronterface.

I have tried this motor on other axis and it does the same thing. Other motors connected to Z work fine.

Do these motors breakdown if put under to much strain?

Martin