I had exactly the same issue. My fix was: Dissasembling the Z-Motor, Electronics (because you can't remove the Z-Rods otherwise), Z-Rods and Headbed. Then i remove the linear bearings from the bed and put a lot of oil into it, and moved the rods a lot of time though them. After that i assembled all back together and oil the rods after that. Also i cleaned the trapzoidal rod and put grease on it. Then i calibrate the bed again and print a small tower to look if the problem was gone, and since that it works all fine again. But i think a problem with linear bearings (i think they a broken) should not happens to a printer in this price category.