Z-Axis Stutters

I'm building a Prusa i3 rework. I'm using Ramps 1.4 from Sainsmart and Kysan stepper with 55 N. I use the repetier 0.91 firmware.
My x and Y axis operate as expected but the z-axis exhibits a strange behavior. When I first boot the printer up after power disconnect and advance the z-axis by 1 mm via the repetier printer panel it will perform the expected 1.2 revolutions equaling 250 full steps. When I try it again it advances only by 20% of a revolution. If I use 10mm it keeps stuttering along like it would execute 10mm steps at 20% revolution.
X and Y work as expected but also have during homing the same effect NOT if I use advance by 100mm.
Anybody can help with this?

Have you checked the current limit on the stepper driver? Voltage may be too low.

Yes i did and you are right. It was too high I turned it down. I thought i had it already almost to the endstop of the pot but aparently not.
I cannot see where it ends really it is too small even under the magnifying glass.
Thanks so much.