The spindle could be too heavy for the motor you have on the Z axis. There are a few things you can do, add a counter weight that will help lift the Z axis. Second and much easier is to add a nitrogen gas shock like on the hood of a car. These come in different lift pressure in pounds. Weight your spindle and get a shock that makes the lift counter act the weight of spindle. Example if you weight the spindle and it is 20 pound get a gas shock that is rated at 20 pounds. This will take a huge load off the Z axis motor and stop drifting down