Have had some issues with the z axis on my 4020 lately, too intermittent to narrow down.
Hadn't even run a program this morning, setting up, servos on, did an M19 to orient the spindle.
Machine sat that way for a few minutes when suddenly I thought someone was pounding on the back door of the shop.
I put my hand on the sheet metal of the head and it was the source of the noise.
Hit the e-stop, then I reset it, hit jog again and realized the head had raised up 0.4"

Anybody ever have this problem?