Update: This is not a speed issue.
I have done everything I can do to test this, and it is just not a speed issue.
V9 post processor has high-feed rates that exceed the recommended speed for the Z axis. I have changed those in my version, and changed all the tool settings for plunge etc.. to try to be sure that there is nothing going out that sends a Z feed that is too fast. Despite this V9 also doesn’t split the code by axis so the feed rate for X/Y is the same as Z and it will go too fast. To fix this I slowed every axis down to 500mm/min and even that is not enough. I am still having this plunge issue.
Clearly this is not being caused by speed. I have also already turned up my stepper’s current a bit and I am still having this problem. I am curious if the people who have not encountered this problem are using the lcd to send gcode or Repetier host?
I have started looking at this more externally to see if I can find a solution.
Same problem on a different marlin based 3d printer
z step lost with marlin again
Marlin lost steps on Repetier forums
so far I don’t have any solution that has stopped my issue completely but I will try to keep posting in case anyone finds anything useful.