K8200 problems while printing

Hello guys,
I built up the k8200 and made my first print yesterday.
But a problem appears when I start printing.
First of all I am using RepetierHost & Slic3er and I can perfectly control everything manually.
But when I am starting a print the amount of extrusion is not right an the printer prints the first layers ok but then it starts to move diagonal and it prints outside of the already printed area.
I have no clue what I can change to fix that so I hope you can help me out.

Hi,

Try changing the reference voltage from 0.425 to 0.55.
This works a lot better.
Check the pulleys on the motors make sure the set screw is tight.

I really think the voltage adjustment will help the best