Tormach Z axis stalling

Hey guys,

I have a Tormach V1 with the white stepper motors..
When I jog the Z axis up at about 20%+ full speed (happens when running programs too. Many bad parts and broken tooling), it stalls out and vibrates a bit. Is the head just too heavy for the old stepper motor, or is this a resonance issue? How can I test either one?

Thanks,
SD