I just checked out my machine. It seems that while the z-axis is supposed to be on hold at a given height, there's a pulse about every 8-10 seconds which causes the motor pully to jog about 1/4 turn. Result is a constant downward progression in steps of between .003" and .005" every few seconds.
What's up with that? Causes what should be a flat machined surface to bcome progressively deeper during the cycle.