Some questions about high-speed stepper motor stalls

Hedda

Hi everyone,
I have a Warco WM18 milling machine and added a powered Z-axis stepper motor, but it's stalling at high speeds.
This is the driver I'm using - https://www.oyostepper.es/goods-111-Controlador-paso-a-paso-digital-10-42A-20-50VCC-para-motor-paso-a-paso-Nema-17-23-24.html
This is the stepper motor - https://www.oyostepper.es/goods-88-Motor-paso-a-paso-Nema-23-bipolar-18-grados-3-Nm-42A-57x57x114mm-motor-paso-a-paso-CNC-de-4-cables.html
This is the power supply - https://www.oyostepper.es/goods-375-Fuente-de-alimentaci%C3%B3n-conmutada-de-150-W-24-V-65-A-115230-V-para-motor-paso-a-pasom%C3%A1quinas-CNC.html
I have the driver set to 4.2A, I think (I can't remember without opening the case), and 1600 steps/rev. It works well, but tends to stall at top speed, especially when the head is raised. Incidentally, the head does have a gas strut assist.
So what did I do wrong?