I had this problem and didn’t actually notice it until today.
Mini Rambo, firmware, and steppers shipped by Ryan.
Today I re-routed my wiring through better plastic conduit, and put everything back together. I also had cut two new Y plates and had them cranked down fairly tight.
Then, at the end of cutting a new part, the router side of my Lowrider v2 suddenly dropped down (lead screw and stepper spinning, collet was not slipping.) At first I thought I somehow messed up the wiring. After restarting and raising Z, it dropped again, and I found I could make the opposite side do the same thing by leaning on it with a little force on top of the motor.
So, it appears that I had two problems:
Y Plate bearing screws were too tight, not allowing smooth up/down motion (power off, the motors should spin and the axis will bottom out)
I have a pretty heavy vacuum hose, and I had reoriented it so there was a few pounds of down force on that side of the Lowrider.
I had seen a few misaligned cuts, with what I thought was X axis skipping, but now I think it was actually from the Z axis collapsing on one side, causing a diagonal cut (oval hole being one symptom.)
Loosen bearings on Y plates so the Z axis moves up and down without hindrance (gravity spins the motors)
Reorient my heavy vacuum hose so the weight isn’t bearing down on one side (10’ shop vac crush proof hose.) I propped it up on a sawhorse for now.
Slowed down Z travel to 2 mm/s
[Edit: See below, had to increase Drive Strength]
So far so good. Made an absolutely perfect part this evening.
Maybe this information will be helpful. I thought the Z axis could lift a lot, but I realized I had quite a bit of weight on it, when you add it all up!