Z axis stepper motor getting lazy....

My Z axis stepper appears to have gotten lazy for some reason. This
stepper is an older slo-syn, 160 Oz-In stepper and is one of three
on the same machine. The X and Y appear to be fine, but the Z axis
has lost most of it's thrust power.

As a comparison, the X & Y axis can push about 80 pounds of force
against a scale before stalling. The Z axis can only push about 30
pounds of force then stalls.

I did disconnect the stepper mechanically to see if there is any
binding etc. and the mechanical motion is free and smooth, the ball
screw / nut rotates nicely etc. so it appears to be electrical.

Anyone have suggestions?

Could it simple be that the stepper is old and ready for the trash
can?

TIA

Chris