Converted to ball screw Z-axis-having issues

Hi
I installed a ball screw on the Z-axis
All was good until I tried to do some lettering involving a v-carve file
It runs fine until the 3d part of the file and then the x&y axis start to stutter.
I reset SB3 to default and the same file ran fine.AS soon as I change the Z-axis unit value to 20780 the file stutters again.
I contact a tech @ shopbot and he informed me that that high of a unit value wont work.
I know others have used ball screws
What am I doing wrong

THX
Brian