Detecting a bad stepper motor

I'm working on a Brigeport retrofit that has NEMA 42 stepper motors on
it controlled by a deskcnc board with Gecko Amps running at 7 amps and
a 50V power supply. Jogging the stages around is fine, there is no
unwanted noise and they move their repetedly and quickly. However
when I start running a toolpath, the z stage sounds like it's missing
counts. The toolpath is the alphabet and my tool is a pen. It's
jumping the z stage up .25" and then down .25" between letters. (I
realize this is probably extraneous, but it's how I came to conclusion
my motor is missing counts because it would try to drive the pen
through the paper).

Is there any other way to test the motor to see if it's bad? The
motors look original and the machine is a solid 20 years old. Do they
go bad easily? Could it be my spindle has to much friction on it? I
haven't gotten the luber working yet, but there is a fair amount of
oil on the spindle shaft.

Any help would be appreciated.

Thanks.

-Fritz