The left Z stepper is loosing steps, either through excessive friction on the left leadscrew or insufficient current through the windings. Are both z steppers plugged into the same stepper driver? Is the z stepper driver getting too hot to touch?
Does it have the same issues if you manually reset the x carriage level at z=0 and drive the x carriage up to Z max?
Can you release the left hand leadscrew nut from the x carriage and manually run it up and down the leadscrew without force? It should spin freely.