Motor Moving Problems

[cobiesmul47]

Hi,
I' sorry i come from Germany and i dont know if my english is so good but i'll try.
We just bought a Geetech Rostock 301 and after solving most of the Problems we now have one we can't solve.
When the Hotend is moving the motor on z-axis randomly clatters(I don't know if its this in English) some millimeters.
We already tried to adjoust the tension of the belt and we gave the motor more Power with the Potentiometer.
It just helped a bit. Maybe someone has the same Problem and can help us!
Thanks in advance!10.0.0.0.1 192.168.1.254