Some question on my stepper motor

Jerry1104

First post and wish i had found this forum a few months ago
They say stepper motor suppliers love newbies
I have a Warco wm18 mill and have added a power z axis stepper but it stalls at high speed
This is the driver i am using - https://www.oyostepper.com/goods-111-Digital-Stepper-Driver-10-42A-20-50VDC-for-Nema-17-23-24-Stepper-Motor.html
This is the stepper motor - https://www.oyostepper.com/goods-493-Nema-17-Stepper-Motor-15A-12V-6374ozin-4-Lead-39mm-Body-W-1m-Cable-and-Connector-for-DIY-CNC-3D-PrinterExtruder-.html
This is the power supply - https://www.oyostepper.com/goods-914-Mean-Well-LRS-150-24-150W-24VDC-65A-115230VAC-Enclosed-Switching-Power-Supply.html
I have set the driver to 4.2a and i think, can't remember without opening enclosure, to 1600 steps/rev
It works quite well but tends to stall at max speed especially raising the head. For info the head does have a gas strut helper
so where have i gone wrong?