Maybe one or more of the screws holding the Z motor in are loose?  I really doubt the motor would flex the sheet metal before skipping or binding itself.  And to do that, the bed would have to not move.

You'd be amazed how easily sometning being out of alignment in the drive can deflect the frame, especially if all the screws are tight. The frame will give long before the motors.

On the few months, I'm printing a lot of big parts and I have seen that when the Z motor down or up the bed,  the surface on the bottom the printer has a deformation just at the moment when the z motor is moving . Who see this effect?
I think that is can a problem with the first layers when you printing.