Thanks to Micromo!
Some had problems of the Z coming down on its own or loosing steps.
a Nema 17 with 76 oz-in/5.47 kg-cm will have a torque left of only 7.45 oz-in/0.54 kg-cm when micro stepping is set to 16! Not so strange that Z comes down with a router that weighs more than that.
Loosing steps is more or less the same story.
A Nema 23 with 175 oz-in has only a torque left of 17.1 oz-in if micro stepping is set to 16. That's not a lot of torque for cutting at feed rates of 1800mm/min and DOC of 3mm. Steps are easily lost that way.
This document might help to make a better choice for the right size of stepper motor for the applications you have in mind.